Set global or chunk fig.showtext = FALSE to keep from setting this option on load

For #36

R/ggplot2.R

   if (!requireNamespace("showtext", quietly = TRUE)) {
     return(invisible())
   }
-  curr_fst <- knitr::opts_chunk$get("fig.showtext")
+
+  curr_fst_chunk <- knitr::opts_current$get("fig.showtext")
+  curr_fst <- curr_fst_chunk %||% knitr::opts_chunk$get("fig.showtext")
+
   if (!is.null(curr_fst) && identical(curr_fst, FALSE)) {
     return(invisible())
   }