Provide a way to access workers in hooks

[whitfin]

This will fix #1. It's just a starting point, but I think this should be sufficient going forward.

This PR adds a :provide option to all hooks, allowing you to ask for various things to be provided to the hook (although currently it's just :worker). This will then call handle_info/2 in the hook with a { :provision, { :worker, worker } } message and allow you to bind the worker to your state.

You can then pass this worker to Cachex (i.e. Cachex.get(worker, "key")) for cache access within the hooks without jumping back over the proc barrier to the worker server.

[coveralls]

Coverage remained the same at 100.0% when pulling 1e95b59 on issue-1 into 6b8f55d on master.