add "selector" parameter to ProxyProvider so state is only recreated when specific variable has changed

[tylandercasper]

Identical request to #764
But he closed it as being to niche, so I wanted to say I would also like this feature.

Is your feature request related to a problem? Please describe.
Basically if ANY value on a base provider changes, than state is rebuilt for ALL proxies that depend on it.

It would be really nice if we could add a filter to the proxy to only recreate if a specific variable was changed.

One of the things I really liked about provider is that the select method allows you to optimize tree-rebuilding based only on what you actually need as opposed to the shotgun style setState({}) a lot of the other ones use.
Not having a similar filter for the providers seems like a missing link to that.