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
Ah, I see now. Confusion all around.

> On further thought I think a simple host match as is sufficient for now.

I was still thinking whitelist, but with simple host matching. Wildcard matching is certainly more extensible (as we do in blacklist), though I figured that is something we could add later if requested.

I think warning by default with an extensible whitelist will allow users to enable block mode and take advantage of the related speed benefits immediately, rather than having to disable block mode and leverage the blacklist if remote requests are required.