Gadget only shows regex functions from installed packages

Fixes #13

R/gadget.R

   )
 )
 
+available_regex_functions <- function() {
+  pkgs <- c("stringr", "rematch2")
+  has_pkg <- vapply(pkgs, requireNamespace, quietly = TRUE, logical(1))
+  regexFn_choices[c("base", pkgs[has_pkg])]
+}
+
 regexFn_substitute <- c(
   paste0(c("", "g"), "sub"),
   paste0("str_replace", c("", "_all"))

R/gadget_ui.R

                 selectInput(
                   "regexFn",
                   label = "Apply Function",
-                  choices = regexFn_choices
+                  choices = available_regex_functions()
                 ),
                 tags$span(
                   class = "help-block",