Support remote assets in Bundle

[Sumukh]

It would be nice to be able to do something like

helper_libs = Bundle('https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.15.1/moment.min.js', 
                                   'static/js/shims.js'
                                    output='public/libs.js') 

Here's my current thinking for how this could work:

• webassets would detect it as remote URL and download the raw file (if it doesn't exist yet)
• the rest of the pipeline would continue and the url of the file for the rest of the process is the download location
• tracking for changes is detecting changes on the URL.

If this isn't a goal of webassets/not a good fit - it looks easy enough to do as an extension/separate library.

I'd welcome implementation suggestions. My current implementation that doesn't touch the internals of webassets creates a subclass of Bundle (WebBundle) and defines the _urls method.

[miracle2k]

URLs are already supported; the limitation is that they are currently always fetched. We don't query for etags or something similar.