Path resolution improvements

[benmosher]

• Need to check for index.js if path is a directory (and node is specified as an environment?)

[benmosher]

Using substack's node-resolve package. Probably need some settings to pass through (extensions, paths, etc.).

[benmosher]

Need a root path, a la webpack's resolve.root. Default to package root.

Extra credit: allow specification of a Node-compatible JS/JSON file with a path (i.e. './webpack.config.js:resolve.root').

May want to spec out some environment-based defaults (webpack, browserify, requirejs, explicitly check for node, etc.)

[benmosher]

closing as completed as initially spec'd, opening new issue for root path settings.