Detect presence of {showtext} as default of `use_showtext` (#56)

Updates color examples and adds left-column headings demo to skeleton

NEWS.md

 
 * Set slide number font size in `rem` so that increasing the font size at the
   slide level doesn't result in giant slide numbers (#53).
+  
+* xaringanthemer no longer sets the `fig.showtext` chunk option when the
+  showtext package isn't installed (#56).
 
 # xaringanthemes 0.3.4
 

R/ggplot2.R

   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 ) {
   requires_xaringanthemer_env(css_file = css_file, try_css = TRUE)
   requires_package(fn = "xaringan_theme")
   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 ) {
   requires_xaringanthemer_env(css_file = css_file, try_css = TRUE)
   requires_package(fn = "xaringan_theme")
 #'   Fonts](https://fonts.google.com)?
 #' @param title_font_size Base text font size, inherits from `title_font_size`,
 #'   or defaults to 14.
-#' @param use_showtext If `TRUE` (default) the \pkg{showtext} package will be
+#' @param use_showtext If `TRUE` the \pkg{showtext} package will be
 #'   used to register Google fonts. Set to `FALSE` to disable this feature
 #'   entirely, which may result in errors during plotting if the fonts used are
-#'   not available locally.
+#'   not available locally. The default is `TRUE` when the \pkg{showtext}
+#'   package is installed.
 #' @param ... Ignored
 #'
 #' @examples
   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 ) {
   text_color <- full_length_hex(text_color)
   background_color <- full_length_hex(background_color)
   text_font_use_google  <- text_font_use_google  %||% is_google_font(text_font)
   title_font_use_google <- title_font_use_google %||% is_google_font(title_font)
 
+  if (is.null(use_showtext)) {
+    use_showtext <- requires_package("showtext", "theme_xaringan", required = FALSE)
+  }
   text_font <- if (!is.null(text_font)) {
     register_font(text_font, identical(text_font_use_google, TRUE) && use_showtext)
   } else {

R/zzz.R

 .onLoad <- function(libname, pkgname, ...) {
 
   if ("knitr" %in% loadedNamespaces()) {
-    knitr::opts_chunk$set(fig.showtext = TRUE)
+    if (requireNamespace("showtext", quietly = TRUE)) {
+      knitr::opts_chunk$set(fig.showtext = TRUE)
+    }
   }
 
   setHook(
     packageEvent("knitr", "onLoad"),
     function(...) {
-      knitr::opts_chunk$set(fig.showtext = TRUE)
+      if (requireNamespace("showtext", quietly = TRUE)) {
+        knitr::opts_chunk$set(fig.showtext = TRUE)
+      }
     }
   )
 

inst/rmarkdown/templates/xaringanthemer/skeleton/.gitignore

+*
+!skeleton.Rmd

inst/rmarkdown/templates/xaringanthemer/skeleton/skeleton.Rmd

   echo = TRUE,
   message = FALSE, 
   warning = FALSE,
-  fig.showtext = requireNamespace("showtext", quietly = TRUE),
   hiline = TRUE
 )
 ```
 
 ## Colors
 
-- <span style="color: var(--text-color)">Text Color</span>
+.left-column[
+Text color
 
-- <span style="color: var(--header-color); font-family: var(--header-font-family);">Header Color</span>
+[Link Color](#3)
 
-- <span style="color: var(--link-color)">Link Color</span>
+**Bold Color**
 
-- <span style="color: var(--text-bold-color); font-weight: bold;">Bold Color</span>
+_Italic Color_
 
-- `inline code color`
+`Inline Code`
+]
+
+.right-column[
+Lorem ipsum dolor sit amet, [consectetur adipiscing elit (link)](#3), 
+sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
+Erat nam at lectus urna.
+Pellentesque elit ullamcorper **dignissim cras tincidunt (bold)** lobortis feugiat. 
+_Eros donec ac odio tempor_ orci dapibus ultrices. 
+Id porta nibh venenatis cras sed felis eget velit aliquet.
+Aliquam id diam maecenas ultricies mi.
+Enim sit amet 
+`code_color("inline")`
+venenatis urna cursus eget nunc scelerisque viverra.
+]
 
 ---
 
 
 ###### Definitely don't use h6 `######`
 
+---
+
+# Left-Column Headings
+
+.left-column[
+## First
+
+## Second
+
+## Third
+]
+
+.right-column[
+Dolor quis aptent mus a dictum ultricies egestas.
+
+Amet egestas neque tempor fermentum proin massa!
+
+Dolor elementum fermentum pharetra lectus arcu pulvinar.
+]
+
 ---
 class: inverse center middle
 

man/theme_xaringan.Rd

   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 )
 }
 \arguments{
 \item{title_font_size}{Base text font size, inherits from \code{title_font_size},
 or defaults to 14.}
 
-\item{use_showtext}{If \code{TRUE} (default) the \pkg{showtext} package will be
+\item{use_showtext}{If \code{TRUE} the \pkg{showtext} package will be
 used to register Google fonts. Set to \code{FALSE} to disable this feature
 entirely, which may result in errors during plotting if the fonts used are
-not available locally.}
+not available locally. The default is \code{TRUE} when the \pkg{showtext}
+package is installed.}
 }
 \value{
 A ggplot2 theme

man/theme_xaringan_base.Rd

   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 )
 }
 \arguments{
 \item{title_font_size}{Base text font size, inherits from \code{title_font_size},
 or defaults to 14.}
 
-\item{use_showtext}{If \code{TRUE} (default) the \pkg{showtext} package will be
+\item{use_showtext}{If \code{TRUE} the \pkg{showtext} package will be
 used to register Google fonts. Set to \code{FALSE} to disable this feature
 entirely, which may result in errors during plotting if the fonts used are
-not available locally.}
+not available locally. The default is \code{TRUE} when the \pkg{showtext}
+package is installed.}
 }
 \value{
 A ggplot2 theme

man/theme_xaringan_inverse.Rd

   title_font = NULL,
   title_font_use_google = NULL,
   title_font_size = NULL,
-  use_showtext = TRUE
+  use_showtext = NULL
 )
 }
 \arguments{
 \item{title_font_size}{Base text font size, inherits from \code{title_font_size},
 or defaults to 14.}
 
-\item{use_showtext}{If \code{TRUE} (default) the \pkg{showtext} package will be
+\item{use_showtext}{If \code{TRUE} the \pkg{showtext} package will be
 used to register Google fonts. Set to \code{FALSE} to disable this feature
 entirely, which may result in errors during plotting if the fonts used are
-not available locally.}
+not available locally. The default is \code{TRUE} when the \pkg{showtext}
+package is installed.}
 }
 \value{
 A ggplot2 theme