Added option to support Streamline '._js' files

Similarly to CoffeeScript support, this patch adds an option to support [streamline](https://github.com/Sage/streamlinejs) `._js` files.

It assumes (as with CoffeeScript) that the user boots Locomotive manually (similar to [this](http://stackoverflow.com/a/14511114/893780)) after having configured the streamline loader (see [this](https://github.com/Sage/streamlinejs/blob/master/examples/loader/loader.md)).