Add support for JSON config files

This pull request adds config file support! You call `.config(key)` to treat the value of `key` as a path to a JSON config file, which is loaded and parsed. The file's properties are added as arguments and are then available for the normal processing/validation that optimist does.

Docs and a test are included.

![](http://f.cl.ly/items/361h0q032w0O0e0v3V39/Notes.png)