Match Ordered Lists

Change `variable.list.gfm` to `variable.unordered.list.gfm`.

Add ordered lists (`variable.ordered.list.gfm`). This change is best illustrated with screenshots:

### Before

![readme md_-__users_andytlr_ atom_packages_language-gfm](https://f.cloud.github.com/assets/475255/2457033/d850d560-af2a-11e3-9baf-2d664c81e7f6.png)

### After

![readme md_-__users_andytlr_ atom_packages_language-gfm](https://f.cloud.github.com/assets/475255/2457028/b738667c-af2a-11e3-930d-fad70b3c2b0d.png)