Customizable options to base rg and ag commands

[isabelgk]

Would it be possible to allow users to customize the rg_base_cmd and ag_base_cmd options? In order to work around #71, I've edited my local copy of search.vim, but it would be nice if I could specify it in my base init.vim instead of in the plugin itself (which can get written over if I update all my plugins).

Thanks for considering.