New option: sort-order-fallback

This PR is based on #161.
Fixes #158 and #160.

If `sort-order` option is set and `sort-order-fallback` is set to `abc`, sort unknown properties (or leftovers) alphabetically.
See tests for examples.

![sad-unipug-271](https://f.cloud.github.com/assets/872004/2232816/b889148a-9b21-11e3-9823-2a4927f789fd.gif)