#11 Support wiki-based paths in media URLs

This would probably be done with `[[media:path/to/whatever.mp3]]` syntax to be supported Later(tm). Basically, make sure that works in URLs! This also means `[[x]]` has to be specifically considered valid as far as data processing is concerned, and we might need to dynamically error/warn if the actual computed value is an invalid (absolute or wiki-relative) URL. I think it's okay to only support either a valid URL or `^\[\[.*\]\]$` - in other places it might be reasonable to concatenate tags, but here it should really only ever be one (i.e. a single tag which returns a URL).