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 think I'm confused. We currently have a white list of local hosts, so we can't use `allow_remote_host`, as it's remote hosts that we're trying to block. Can you explain a little more?