Rx'ify ignore and config modes
Fix quoting
[Fix #20] Don't font lock Magit header before Magit is loaded

`git-commit-skip-magit-header-regexp` is nil before Magit is loaded.  If
Git Commit Mode was loaded first, the variable would be added as `nil`
into the font lock keywords, thus breaking fontification.
Release Git Commit Mode 0.13
Bump version of Git Commit mode
Document the move to Magit