Add support for url() values in css

[xt0rted]

Checklist:

• If this PR is a new feature, please provide at least one example link
• Make sure all of the significant new logic is covered by tests

Here's some examples of this working.

• https://github.com/bhaumik-sassyinfotech/uz/blob/080829048a62bd38434fcf132b3d2ee0ed6732c8/secure-admin/assets/js/ckeditor/uploader/filemanager/scripts/jquery.filetree/jqueryFileTree.css#L28-L284
• https://github.com/Deluxcode-paul/Laravel-5-Sample-Code/blob/4ee8271655cd635872922a88e5e3332bbf3e9614/public/vendor/flex-cms/js/vendor/AjexFileManager/skin/dark/dark.css#L176-L186
• https://github.com/FreSemin/FreSemin.github.io/blob/c05f10febc17adbe9f81f04012ec370097774f11/KENIGZ/scss/style.scss#L443

This is a feature that'll never work for everyone since the assets could be anywhere in the repo, but it does handle the basics which should be good enough.

I still need to add some tests for this and some feedback on the adjustment to the sass plugin.

Closes #447

[stefanbuck]

This looks straight forward to me. As mentioned on the issue, ideally OctoLinker would be able to resolve all relative files regardless of the language, but in the meantime this is defiantly better than nothing.