Attrs through fragments

[plexus]

Implement setting attributes through fragment syntax

See the corresponding notebook for full explanation and examples.

This also means we now expand :<> ourselves (when used directly inside a component), so you can now tap into this even if your Hiccup implementation is not aware how to deal with this.

Added a notebook that explains things in details.

Still needs

• [x] tests
• [x] changelog
• [x] readme

[plexus]

FINALLY wrapped this up. @oxalorg you want to give this a once-over?

[plexus]

@oxalorg I'm all ready with this but seems this needs a rebase.