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
This sounds great. My reservations with the current implementation are in part due to the overlap with the blacklist functionality, while being less extensible. Such an API seems like it would be extensible enough for our users and also allowing simple blocking of external requests for the standard case.