Add support for "XXX"

Yea? Nay? Vim has it out of the box:

![screen shot 2014-03-18 at 10 05 02 pm](https://f.cloud.github.com/assets/4397642/2455889/436442c6-af0b-11e3-915b-251b9af07804.png)

Personally, I use `XXX` as a semantically shorter-term marker than TODO. :smile: