Add support for LiveScript files

CodeMirror already support this, so it's the only required step to make Brackets support it natively.

There's [an extension](https://github.com/devinceble/brackets-livescript) which does that, but it does an extra step by [defining the language](https://github.com/devinceble/brackets-livescript/blob/master/main.js), what [is already done](https://github.com/marijnh/CodeMirror/blob/master/mode/livescript/livescript.js) by CodeMirror.

In this way bugs fixed by CodeMirror need to be applied manually in the extension, which isn't maintainable. On the other hand adding native support for LiveScript will resolve this problem as Brackets is getting CodeMirror updates automatically.