Add optional (fallback) grepformat to g:ackformat

`grepformat` can hold multiple values separated by commas. When parsing results Vim tries to match lines against each of formats specified.

I added additional format `%f:%l:%m` that doesn't include column. This allows using `git-grep` (that don't have an option to output column number) with this plugin just by setting `g:ackprg` like this:

    let g:ackprg = 'git grep -H --line-number --no-color'