Add a new 'subscribe' command which allows people to subscribe for
automatic notifications via direct message of topics which match the
subscription regex they provide. With no regex argument it shows the
user's current subscription (if any). Also add a new 'unsubscribe'
command for clearing the regex.
Example use cases:
1. I know that nova is planning to discuss $TOPIC some time tomorrow
but they don't know exactly when, and I want to spend most of the
day in another room whilst ensuring I don't miss that particular
discussion on $TOPIC => "/msg ptgbot subscribe $TOPIC" will give me
notifications when the PTL types "#nova next $TOPIC" and "#nova now
2. I'm interested in *all* discussion on Python 3. I don't know which
projects are planning to discuss it, let alone when, but that
doesn't matter, because I can type "/msg ptgbot subscribe python ?3"
and get notified of all Python 3 discussions.
As with the presence tracking commands, these commands can be used in
public channels by preceding them with a '#' character.