Make it work with --group about ag.vim

[losingkeys]

Is the desired behavior:

1. To have a blank line in the quickfix window between matches for different files
2. To see the same blank line in the scrollback when you background/quit vim
3. Both
4. Something else

?

[losingkeys]

--group is the default, right? If so then it works out of the box with --group. If not, can you provide an ag --some-options command (that you'd run outside of vim) that causes this to fail? Once #27 is merged you'll be able just to set g:agprg to some ag command with options and g:agformat to something that the default errorformat option accepts, and it should just work.