Port plugin to ES6

[theefer]

Proof-of-concept attempt to convert this plugin from AMD to ES6 module.

Note:

• As I was going to reindent the code, I ported it to 4-spaces, which I think we wanted to make standard across the codebase?
• The output AMD module (from plumber build) looks a bit funky, with ES5 getter based defaults. From what I can Google, that should be supported by all modern browsers (but excluding IE7-8).
• It'd be good to prove that the output of traceur.toCommonJs() works fine for people who live in browserifyland.

[TooTallNate]

Pretty cool :)

[OliverJAsh]

You can remove this now!

[OliverJAsh]

• If we're going to move to a tab width of 4 spaces, are we going to port all Scribe modules over? In any case, we need to add/update .editorconfig with this information.
• Can we test the output works without any changes with an example, e.g. the one in the Scribe repo?
• Can we test the new source maps work as expected?