feat: Add option to check buftype option before enabling

Closes #101

Instead of only having a big list of "naughty" filetypes, have an option
to check for the `buftype` and skip if set to anything.

autoload/indent_guides.vim

@@ -284,6 +284,11 @@ endfunction
 " Detect if any of the buffer filetypes should be excluded.
 "
 function! indent_guides#exclude_filetype() abort
+  if exists('g:indent_guides_exclude_buftype')
+    if g:indent_guides_exclude_buftype && &buftype != ''
+      return 1
+    endif
+  endif
   for ft in split(&ft, '\.')
     if index(g:indent_guides_exclude_filetypes, ft) > -1
       return 1