Warn about remote requests with option to block

Adds a new request filter with three modes:

* Warn on an outgoing remote request (default)
* Stub out remote requests with an empty response
* Allow remote requests without a warning
* Allow adding custom localhost definitions
I was trying to keep this really simple and straightforward for the intended use case: dealing with external requests from unit tests. I think the blacklist was created for a more general-purpose audience and set of use cases, and I think this new feature will replace it for most users.