feat: add search_dirs to split_keywords table (#986)

This allows the command parser to correctly split `search_dirs` into a
table. Should solve nvim-telescope/telescope.nvim#985

lua/telescope/command.lua

@@ -123,7 +123,8 @@ end
 local split_keywords = {
   ['find_command'] = true,
   ['vimgrep_arguments'] = true,
-  ['sections'] = true
+  ['sections'] = true,
+  ['search_dirs'] = true
 }
 
 function command.register_keyword(keyword)