Instruct git to ignore the vim/spell directory
Adjust tmux message line colors
Adjust some formatting options in .vimrc
Fix typo in .vimrc
Apply better fix for tmux pasteboard issue in OSX

This fix requires some conditional configuration in tmux so that the fix
gets loaded only in OSX, and not in Linux systems, for instance.  This
commit uses a better solution for this conditioning, than what it was
being done before.