Feature request: define inaccessible destinations

Currently a client connecting to muonproxy running on a remote server can perform connections to ports bound to 127.0.0.x addresses on that server, for example: ``` curl -x socks4://remote.host:1080 http://127.0.0.1:9999/ ``` will send an HTTP request to the service listening on 127.0.0.1:9999 on `remote.host` that would not otherwise be accessible. This could potentially be surprising for the server operator, and/or a security issue. It would be nice to be able to define a list of ranges of addresses that should not be accessible via the proxy service, possibly with 127.* addresses being on that list by default.