Move babel specific options into options.babel property.

[rwjblue]

Currently we try to treat this packages options as both Broccoli specific options and Babel's options. This is quite painful (passing another option to the Broccoli plugin is actually a breaking change for the babel transpilation side of things) and not terribly useful.

We should refactor our incoming options to move the babel specific options into options.babel so that we can delete the following munging of options:

broccoli-babel-transpiler/index.js

Lines 60 to 70 in ff975c0

	delete options.persist;
	delete options.async;
	delete options.annotation;
	delete options.description;
	this._optionsHash = null;
	this.console = options.console || console;
	this.throwUnlessParallelizable = options.throwUnlessParallelizable;
	delete options.console;
	delete options.throwUnlessParallelizable;

[nlfurniss]

I can take this on in a separate PR