Preprocessor options

This adds a new option `preprocessOpts` which gets passed through to the preprocessor.

Since I'm after the `root` option on the preprocessor, I used it for the tests. `importer.css` imports `importee.css` in the `test/fixtures/preprocess` directory with a simple `@import "./importee.css";`

I set the `preprocessOpts.root` option to `test/fixtures/preprocess` in the Gruntfile, and thus allowing `rework-npm` to correctly import the relative path `./importee.css`.

Fixes #5