Module resolution root path setting

[benmosher]

Need a root path, a la webpack's resolve.root. i.e., if .eslintrc has something like

---
settings:
  resolve.root: 'src'

...then the resolver for resolve.js should look in path.join(process.cwd(), 'src') as an additional base path.

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.)

Need to investigate proper setting key conventions (i.e. import/resolveRoot?) within ESLint plugins.