mechanism for adding peers manually

Some toronto mesh folks (https://github.com/tomeshnet/prototype-cjdns-pi/issues/213) would appreciate this, since it would let them bypass dht and lan routing (since they're usng cjdns) and just connect to known peers directly.

This could be stored in a `${CONFIG}/cabal/config` json file, or similar. [cabal-client](https://github.com/cabal-club/cabal-client) would also be a good place for this.

Start by reviewing the cabal-cli and cabal-client entry points for peer discovery and connection setup, then compare them with the linked cjdns use case. Decide how the proposed `${CONFIG}/cabal/config` JSON would be read and how manually listed peers would bypass DHT and LAN routing; done means known peers connect directly through the CLI.

Written by the indexing model from the issue text.