Highlight code blocks as their native language.

See #3. This is basically cargo-culted entirely from the way [the GFM language does it](https://github.com/atom/language-gfm/blob/master/grammars/gfm.cson#L248-L264).

## Language List

ReStructured Text uses [Pygments](http://pygments.org/docs/lexers/) for syntax highlighting, while the languages Atom/GFM support can be lifted from [the GFM plugin](https://github.com/atom/language-gfm/blob/master/grammars/gfm.cson#L146-L504).

- [x] Pygments: `coffee-?script` => Atom: `source.coffee`
- [x] Pygments: `js|javascript` => Atom: `source.js`
- [x] Pygments: `json` => Atom: `source.json`
- [x] Pygments: `css` => Atom: `source.css`
- [ ] Atom: `source.less`
- [x] Pygments: `xml` => Atom: `source.xml`
- [x] Pygments: `rb|[rd]uby` => Atom: `source.ruby`
- [x] Pygments: `java` => Atom: `source.java`
- [x] Pygments: `erlang` => Atom: `source.erlang`
- [x] Pygments: `csharp|c#` => Atom: `source.cs`
- [x] Pygments: `php[3-5]?` => Atom: `source.php`
- [x] Atom: `source.shell` *(Maybe the bash lexer?)*
- [x] Pygments: `py(thon)?|sage` => Atom: `source.python`
- [x] Pygments: `objective-?c||obj-?c` => Atom: `source.objc`
- [ ] Atom: `source.basic` => *(is this VB.net?)*
- [x] Pygments: `yaml` => Atom: `source.yaml`