add support for additional_import_paths and add_import_path

These are available in the [compass configuration file](http://compass-style.org/help/tutorials/configuration-reference/).

I've added support for using these to create additional watchers to provide parity with `$ compass watch` (in this regard).

`additional_import_paths` is an array of paths (as strings) and `add_import_path` is a method to append a path (as a sting) to this array so they are both supported by enumerating `::Compass.configuration.additional_import_paths`