Allow setting the 'errorformat' option for Ag.vim

- this is used in parsing ag's output
- see `:h errorformat` for more info
- when this can't be set, then certain ag options cannot be used in
  `g:agprg` (you won't be able to jump to the matches because the quickfix
  list won't be populated correctly if they do)

I don't see an issue with forcing the `'errorformat'` when we're searching for files (especially because this is a  command the plugin provides), but we could wrap that too if you could somehow get different output when using the `-g` flag.