Implement a 'remote_host' matcher

[cohesive-flight]

I would like to see this module include a 'remote_host' matcher, similar to the one in the caddy-remote-host module. I'm running this within docker, so it'd be preferable to refer to the containers based on their names instead of their IPs.

Please also consider adding an option to include the hostname in the debug logs rather than just the IP.

[mholt]

The problem with that is it requires expensive DNS lookups in a hot path.

[cohesive-flight]

I suggest adding a disclaimer for the matcher then. It would make it up to the user if they're willing to take a performance hit for the convenience it'd provide. And in docker's case, since the records are defined in an internal DNS server, it should help minimize its impact.

[mholt]

That may be fine; PRs welcome.