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
Okay, I'll give this another look. If we're not treating it as specific to remote requests, I'd like to:

* Update naming to remove references to "local" or "remote"
* Support the same syntax for the whitelist
* Use the same API for both whitelist/blacklist
* Re-use the same modes for the blacklist, so you can receive warnings for blacklisted requests instead