Added ability to set the sprite load path in the Compass Filter

There is another issue ongoing which would remove the requirement for this PR (https://github.com/kriswallsmith/assetic/pull/314) - but until that issue has been further discussed this configuration option lets you set the sprite_load_path and image_dir independently.

There are multiple issues and questions arising about sprite_paths and with this config option I have a pretty decent solution which answers most questions I've seen. I'd be able to document this in the SF2 cookbook once the PR is merged.