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 was thinking we'd want to write directly to stderr instead of to the logger, because users don't generally run in `:webkit_debug` mode, but I think we always want them to see these warnings.

At some point, we may want to replace `:webkit`/`:webkit_debug` with a log level setting.