Add key to data-on-signals-change

[bencroker]

It might be useful to be able to listen for when a specific signal changes, for example:

<div data-on-signals-change-foo="alert('foo changed')"></div>

[delaneyj]

this need more thought, what about if you want more keys? would an event be better instead? This seems like a discussion for now

[bencroker]

My thinking is that data-on-signals-change-foo would be triggered whenever foo or foo.bar changes, allowing you to listen on nested signal changes. If you want another key, just add another attribute (data-on-signals-change-baz).