Adds support for the `compass compile -I IMPORT_PATH` setting.

Using -I we can specify an additional folder which will then be made available to our Sass via the @import function.

Updates the readme file to reflect this added support.
Only add the `-I` setting to the options object if actually set.
Merge pull request #6 from jannisg/feature/import-path-support

Support for Compass' compile -I IMPORT_PATH option