Add GFM todo snippet

I originally tried something that would let a second tab jump into the `[ ]` box, but it proved to be buggy; tabbing from a subsequent item would jump back up to the `[ ]` box from the previous line:

```coffee
'body': '- [${2: }] $1'
```

So I went with something simple as I doubt many people are writing already completed todos anyway.

### In JIFF form:

![lol](https://f.cloud.github.com/assets/475255/2479227/a24771f4-b092-11e3-8e7d-9a96c7fa6408.gif)

### Tests

There doesn't seem to be existing tests for any snippets, so I haven't written any for this. Happy to if I've missed something.