Observable collections

[jayvdb]

I would like to have observable dicts to notify on item add/remove. This seems like a pattern that observable might want to provide an implementation for, and/or docs with a recipe for observing core collection types. If the core types need to be subclassed in order to be observed, a helper/mixin would be useful to make it easy.

[timofurrer]

Sounds like an interesting idea. I might get to it when I've some time.

I'm always happy to review PRs though, if you are up to it 🎉