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'm concerned about the semantics of "allow_remote_host," since I think it would usually be used with a new local which should be consider local, like `::1` or something else in the `127.0.0.255` space.