Added config options to spell check all git commits, and change the
default highlight style of misspelled words to use underline.
Mix of tips from
http://stackoverflow.com/questions/6008921/how-do-i-change-the-highlight-style-in-my-vim-spellcheck
and
http://stackoverflow.com/questions/1691060/vim-set-spell-in-file-git-commit-editmsg