Allow renaming contacts

[exquo]

Signal-cli has an updateContact method that allows to change contact's name. It might be useful to be able to do this from scli. This command can also be used to create a new contact.

Like #72, this is mostly a matter of adding the UI. I don't know what would be the most natural way to do it, maybe have a pop-up 'edit contact' dialog bound to some keypress (e.g. e) when the cursor is over the contact's name? Or have a :renameContact command?

Signal-cli's updateContact only changes names locally, in its own database. It needs a sendContacts call to sync it with server. Maybe this should be added as an addiotonal button in the 'edit contact' pop-up window.

[isamert]

I think a popup will complicate things. An :renameContact command would be a better fit. If commands become more discoverable, it would be even better. There is already a basic command completion with Tab key and basic contextual completion, like :attach ~/<TAB> would list files under your home folder. Likewise, :renameContact <TAB> would list all the possible contacts with incremental completion and the second argument would be the new contact name.

[exquo]

It needs a sendContacts call to sync it with server.

From the sendContacts section of signal-cli's man page:

This command should only be used if this is the master device.

So if signal-cli is not master (e.g. has been linked with signal app on the phone), there is no way to update the contacts list on signal servers from signal-cli / scli..