More options

We've been using this alongside rework-npm at [recurly.com](http://recurly.com). We extended it to meet our actual use case and suspect others may benefit from these additions:

- removed assumption of `build` directory from css output (since you're usually not building to somewhere inside of your source directory)
- aligned options with other node style parsers like stylus (`src`, `dest`)
- added a `func` option with which you can specify the css function to override. For example, we want to keep some of our urls un-transformed, and we'd like to be explicit rather than magical about what's happening to our designers' source files. So `asset()` is used for bundled assets and `url()` is used for urls that should remain unmodified.
- updated tests to match
- updated readme