Add support for Proc args to consumer key/secret

Allows the use of a Proc as a `consumer_key` and `consumer_secret` argument when initializing an OAuth strategy. This Proc receives the Rack env as an argument, allowing variable configuration based on things like a what domain it is being called on.

Wrote this up to solve a problem for a site I'm working on that requires authenticating against different endpoints depending on what domain the request comes from. Feedback greatly appreciated!