add options; implement strict_missing

(upstream of https://github.com/venmo/btnamespace/pull/3)

assign: @amygdalama 

Currently, non-namespaced resources created before entering a namespace are accessible. ie:
  1. create a resource 'foo'
  2. activate a namespace
  3. look up resource 'foo'. btnamespace will log a warning that it doesn't recognize this id, but let the request continue through to the client library

This adds an option to shortcircuit in step 3 and immediately raise `braintree.exceptions.NotFoundError` (what the client library raises on a 404).

I have it off by default right now, but we might want to make it the default behavior.
Yeah, yagni is fair here. My thought process when implementing this:
* I want to set some action-specific config on the namespace
* I may as well make it generic (arbitrary dict instead of passing, say, `strict_passing=bool` to actions)
* We don't use options inside hooks right now, but I could imagine cases where that'd be useful. we do technically support custom schemas (even though probably nobody uses those right now).