Feature: haml coffee option

Adds the option to specify haml-coffee as the language compiler through `hamlPreprocessor` options:
```js
// karma.conf.js
module.exports = function(config) {
  config.set({
    ...
    hamlPreprocessor: {
      options: {
        language: 'coffee'
      }
    }
  });
};
```

The default behaviour is unchanged, thus backward compatible.