Adding ability to output ES6 JS blocks for Firefox testing

This should allow you to create JavaScript jsbin's which output with a different `mimetype` that firefox needs to run ES6 code.

You should be able to put this code in the JS tab and get it to execute:

    var fn = (x) => x;
    console.log(fn(42) == 42);

Basically it just outputs the right `type` on the script tag