Match Mentions

This matches mentions:

![screen shot 2014-03-26 at 9 57 20 am](https://f.cloud.github.com/assets/475255/2519424/8d559734-b471-11e3-9eae-ab94518d86d8.png)

### Couldn't work out

As mentioned in the screenshot; @names that are at the start of both ordered an unordered list items don't match. Presumably because it's right next to the `1.` or `*`, but there is whitespace between them which this regex matches against.

### I wasn't sure about

Using `variable` for the @ and `string` for the username. Perhaps the username could be `bold`. But neither seem semantic. I picked `variable` and `string` because it looks like they are used in other places so that any syntax colour scheme can be used.

### Follow up

As another PR I was thinking it'd be nice to match issues like #1 using a very similar regex.