Uses :filesystem_importer sass option, when provided, to create importers

Sass allows the user to overwrite the default importer using :filesystem_importer option. This makes Compass respect that option when used.

I didn't added any tests because, honestly, I don't think I could do a decent job without putting a LOT of effort to it, and this is a very small modification.

I'm starting a new OSS project called Liftr and I created a custom importer for sass which only imports a file once (https://gist.github.com/4549615) but I need to be able to set the `:filesystem_importer` option (supported by sass) for it to work. I think this import would actually be a nice addition to compass or even sass.