CoffeeScript support

With this pull request `browserbuild` can compile CoffeeScript automatically, including `.litcoffee` and `.coffee.md` files. E.g.:

```
$ cat main.litcoffee
Let's print:

    console.log 'bla!'

$ bin/browserbuild -m main main.litcoffee
...
require.register("main.js", function(module, exports, require, global){
// Generated by CoffeeScript 1.6.2
(function() {
  console.log('bla!');

}).call(this);
...
```