fix yasnippet exit hook

This will preserve the intent of issue #51 (highlight and indent after yasnippet insert) without clobbering any custom/manual indention in the buffer due to the full-buffer reindent.  

This in turn resolves #122, without requiring users to remove the hook (and revert to non-indenting/highlighting snippets) in their emacs config.
Merge pull request #141 from edslocomb/fix-yasnippet-hook

fix yasnippet exit hook