Plugin support for static files and staticdirect

[Christopher Allan Webber]

Plugins need a way to add their own static files, like CSS, images, etc. They also need ways for them to be linked and statically served. Then they also need a way to include those files in templates.

The docs make some mention of this... the assetlink command we have can be extended for this, and I think we can both extend staticdirect to take a "domain" like:

  {{ request.staticdirect('/images/foo.jpg', domain='plugin:someplugin') }}
  {{ request.staticdirect('/images/foo.jpg', domain='theme') }}

This could also have fallback support... it would be good maybe to try to do something like this:

  {{ request.staticdirect(
         '/images/foo.jpg',
         ["__theme__", "someplugin", "__main__"]) }}

That above example would first try assets from the theme, then someplugin, then __main__. It would be good if the location where these things were found would be cached so as to not have to do more checks for os.path.exists() than we have to.