fix up comment highlighting

- TODO (and its variants) now highlight as normal
- Nested comments now work
- Spelling is checked in comments (and not in code)

The method of getting nested comments to work (adding `matchgroup=...`) is still a bit magical to me.  [The documentation](http://vimdoc.sourceforge.net/htmldoc/usr_44.html#44.7) didn't help very much... but it works :) .