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)
Merge pull request #27 from joshuarh/allow-setting-errorformat

Allow setting the 'errorformat' option for Ag.vim