Figure out how to support jsnext:main in webpack

[sohkai]

For other libraries that rely on this library, this is actually super important as their CommonJS exports won't work if they import directly from /es6 directories of a library.

webpack/webpack#1979 Has a good rundown on what to do, but when trying it with the latest webpack 2 version, it doesn't seem to be picking up jsnext:main properly.