Add support for jsnext:main

[dandv]

...and/or pkg.module - see diegohaz/nod#24

[jsynowiec]

After reading the explanation in jsforum/jsforum#5 I'm not sure if this should be included in this boilerplate. It's up to the authors to decide whether to include pkg.module or not.
Moreover, ES6 code with Flow annotations is not a valid ES6. It needs to be processed first (or some on-the-fly loaders are required).