Setting Babel module format

In [Babel](http://babeljs.io) you can set which module format should be used. Default is CommonJS. You can set amd, system and umd as well.

``` sh
$ babel -m amd index.js
```

It would be nice if we could set this in the settings pane of `atom-preview` so preview can display the different outputs depending on this setting. I tried to make a pull request for this but unfortunately I am not experienced enough to accomplish this :unamused:

Start by locating the atom-preview settings pane and the code that invokes Babel for previews. Check how the existing module format is selected, then add a setting for CommonJS, AMD, System, and UMD; done means each choice produces the corresponding preview output.

Written by the indexing model from the issue text.