Imports component

Hi,
I added the ability to import another components stylus file. For instance https://github.com/kewah/component-stylus-plugin/blob/imports-component/test/fixtures/with-import/index.styl

I also added tests.

Let me know if I broke something from your previous version. Without tests it's a bit complicated ;)

One downside with this implementation is, we must be careful to not import files containing CSS properties, otherwise they will be duplicated in the output.

Related to #6