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'
Fix q mapping for location list commands

Variations of :LAck had 'q' mapped to :cclose, which would never trigger
the location list to close.
Add auto-preview feature

Auto-preview files when they are selected in the quickfix window.
Include `let g:ackpreview=1` in your .vimrc to enable.
handle warning for conflicting arguments in ack 2.0

the -g argument can not be used with the -H or --column arguments, so we
remove them
Add AckWindow/LAckWindow to search buffers displayed on current tab page.
Improved handling of filenames for :AckWindow
Fix duplicated results for files displayed twice
Fix go mapping
Issue fix #84

Fix to highlight works with quotes and doubles quotes
Update wording in the helptag
Added 'O' Shortcut

Added 'O' (Upper case 'o') shortcut to open the file and close
the quickfix window.
Escape `#` and `%` passed to &grepprg

Fixes #67
Merge pull request #89 from kassio/master

Issue fix #84
Merge pull request #58 from sickill/patch-1

Add optional (fallback) grepformat to g:ackformat
Merge pull request #63 from rizzatti/close_location_list

Fix q mapping for location list commands
fix ackhighlight
Merge pull request #97 from blueyed/master

Escape `#` and `%` passed to &grepprg
Merge pull request #95 from ilyakatz/doc_occurance

Minor tweak to documentation to correct previous wording of "error"
Merge pull request #96 from imtapps/master

Added 'O' Shortcut
fixes issue #81 and closes #99

thank's for @MultiCoreNop, who open a PR with the fix