add "append" arg to `write_extra_css()` (#20)

add "append" arg to `write_extra_css()`

R/write_extra_css.R

 #' @param css A named list of CSS definitions each containing a named list
 #'   of CSS property-value pairs, i.e.
 #'   `list(".class-id" = list("css-property" = "value"))`
+#' @param append If `TRUE` output will be appended to `outfile`; otherwise,
+#'   it will overwrite the contents of `outfile`.
 #' @inheritParams write_xaringan_theme
 #' @export
-write_extra_css <- function(css, outfile = "xaringan-themer.css") {
+write_extra_css <- function(css, outfile = "xaringan-themer.css", append = TRUE) {
   cat("\n\n/* Extra CSS */", list2css(css), file = outfile,
-      append = TRUE, sep = "\n")
+      append = append, sep = "\n")
 }
 
 #' @inheritParams write_extra_css

man/write_extra_css.Rd

 \alias{write_extra_css}
 \title{Write Extra CSS arguments}
 \usage{
-write_extra_css(css, outfile = "xaringan-themer.css")
+write_extra_css(css, outfile = "xaringan-themer.css", append = TRUE)
 }
 \arguments{
 \item{css}{A named list of CSS definitions each containing a named list
 \code{list(".class-id" = list("css-property" = "value"))}}
 
 \item{outfile}{Customized xaringan CSS output file name, default is "xaringan-themer.css"}
+
+\item{append}{If \code{TRUE} output will be appended to \code{outfile}; otherwise,
+it will overwrite the contents of \code{outfile}.}
 }
 \description{
 Manually adds css elements to target \code{outfile}.