enhanced-resolve as underlying Webpack resolver

[benmosher]

In trying to support the end goal of #320, this PR uses the underlying Webpack enhanced-resolve package (and the actual plugins thereof) to resolve under Webpack.

Additional work that would be ideal:

• reference node-browser-libs, enhanced-resolve via peer dependency with webpack
• use WebpackOptionsDefaulter from webpack peer to initialize defaults
  • maybe in the future, don't want to figure out how to find webpack from eslint
• plan support for Webpack 2 (i.e. use ResolverFactory from enhanced-resolve 2.x)
  • should be a sibling package, I think. at least to start