Feature Request: Facebook messenger style nicknames

One of the most fun features of Facebook Messenger is nickname support, where anyone in a channel can set anyone else's nickname to whatever they want. I know it sounds stupid, but this is endlessly entertaining and I'm not sure any other messaging app does it. I think this would help Keybase Chat be more appealing to family and friend groups.

I think it could be implemented similarly to the existing `/headline` feature, something like `/nickname @wilg Butt Face`. Perhaps it should be optional or something?

Start by locating the existing /headline feature in the Go client and tracing how chat commands and displayed names are handled. Define the permission and optional-behavior questions before implementation. Done should allow a channel member to set another participant's nickname with a command and show that nickname consistently in the channel.

Written by the indexing model from the issue text.