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 could imagine the desire to enable `block_remote_requests` as an alternative to the blacklist, whitelisting single hosts as desired. One would set `block_remote_requests` and then allow the desired remote host. This would work with `add_locahost`, but the naming doesn't make much sense. I'm proposing an alternate name of `add_localhost` which would work for this use case.

Really, it's just a naming issue, expanding the possible uses of this feature. Either way I'm fine with it, but I'm just trying to imagine how this may be utilized by our users.