Move lib/defaults into ./overidify-rules.js
Re-order/re-write the error handling.

Now presents better error messages to the user, takes defaults with warnings.
Add filtering based on file extension. This is the default for node does, though we don't handle javascript-y file extensions well (e.g. .coffee).
Load in options from environment variables.

Uses $OVERIDIFY_PREFIX as a prefix to a config key in the rules.js file.
Expose file resolver as a 'resolve' method on the overidify module.
Merge branch 'master' of github.com:jhugman/overidify