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
In other commands, we write to the `logger` provided by `m_manager`. I would paste a link to line 17 in `PageLoadingCommand`, but Chrome has decided it no longer wants to allow copy or paste from text fields.