url-property: add url as a property that can be determined from the app context

This repeats the hack that's done fore `directory` to `url` so that filters that use `self.ctx.url` work, as it is in the [Compass filter](https://github.com/miracle2k/webassets/blob/master/src/webassets/filter/compass.py#L191).
I think this needs to refer to 'url'.