stylus.url can now timestamp too large images

If an image was considered too large to base 64 inline (according to `limit`), its url can be timestamped with a query string instead, so that larges files can be cached with a far-future expires header, but still be updated when modified (cache bust).

I added the logic, and documented it. I couldn't find how to write tests for it though.

Please tell me what you think, and what could be improved.