option for caller ID via /me command

Many XMPP clients have poor support for _XEP-0172 User Nicknames_ and _XEP-0054 vcard-temp_ which we use for caller ID. In some cases they lack support completely. In other cases the UI is bad. It would be nice to be able to fall back to _XEP-0245 /me Command_ as a last effort to communicate this information. Since _/me_ is handled via normal chat messages, it should work with every XMPP client.

Support should be hidden behind an option (default off) because it generates message content which the SMS user didn't create themselves. Many users might not want that sms-over-xmpp injecting content into their chat stream.

* [ ] track time of most recent message to/from a contact (new fields in `xmppContact`)
* [ ] add "caller-id./me = yes" option
* [ ] send "/me is John Wayne" as first message if no previous message exists

This is a follow up to #9. It would benefit from the longevity described in #19