Local Server Push + push intermediates without routes

[d4l3k]

This adds support for local server pushing instead of just setting the Link header via WithShouldPush().

This also allows for pushing intermediate files without the routes specified. The old behavior was if you had a route "/foo": "my-foo.html" and you just requested my-foo.html nothing would be pushed. It now pushes on either case which enables most of the benefits of push for routes that aren't known ahead of time.