Added support for highlighted TODOs in SCSS files

Hi there,

Just added the support for highlighting TODOs in SCSS files. Those keywords (``TODO``, ``FIXME``, ``NOTE``, ``OPTIMIZE``, ``XXX``) are linked to the vim ``todo``class. It's quite handy to be able to quickly spot TODO items in the code.

* Added a keyword line, specifying that those keyword are always contained, either in scssComments or cssComments
* And linked ``scssTodo`` to vim default ``todo`` class

And a quick preview... 
Before :
![before](https://f.cloud.github.com/assets/769918/369565/c6d7f5e6-a2da-11e2-8755-6adb8000accb.png)
And after : 
![after oneliner](https://f.cloud.github.com/assets/769918/369566/cfa558a8-a2da-11e2-8617-11b9d295a966.png)
![after nested](https://f.cloud.github.com/assets/769918/369567/d14b6170-a2da-11e2-8e4b-e2391d0931c0.png)

Cheers,
Paddy