implement invite to channel

[cblgh]

if we have a method that takes a user name and returns a list of public keys i'm thinking it can be used for a facility that allows for inviting users into channels e.g.

   /invite <name> <list of channel names>

extracted from #16 :

/invite where pubkey is the peer i want to invite to

or /invite where every peer that currently has the name is invited
multiple peers should be able to be invited at the same time too, and with autocomplete for nicks