Don't set `fig.showtext` if it's already set to `FALSE`

Fixes #36

NEWS.md

   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).
+  showtext package isn't installed and also when `fig.showtext = FALSE` 
+  (thanks @mikedecr #36, #56).
  
 * The style functions gain a `link_decoration` argument to set the
   [`text-decoration`](https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration)

R/ggplot2.R

   ))
 }
 
+set_fig_showtext <- function() {
+  if (!requireNamespace("showtext", quietly = TRUE)) {
+    return(invisible())
+  }
+  curr_fst <- knitr::opts_chunk$get("fig.showtext")
+  if (!is.null(curr_fst) && identical(curr_fst, FALSE)) {
+    return(invisible())
+  }
+  knitr::opts_chunk$set(fig.showtext = TRUE)
+}
+
 requires_xaringanthemer_env <- function(
   css_file = NULL,
   try_css = TRUE,

R/zzz.R

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