[polybar] Add pomo timer as git submodule

Using [jsspencer/pomo](https://github.com/jsspencer/pomo) as the timer
of choice. It is simple, it works well, and it uses a very elegant way
of tracking the time.

I can think about implementing my own version later, but this will
handily suffice for now.

Polybar script will now limit the pomo timer to a single 'running'
instance containing the notification function, by putting its running
instance PID in a lockfile and killing it before restarting or changing
the timer.

Added running and paused tomato icons; as well as break coffee icon.
Removed display of seconds, unless the last minute of a timer is
running.

polybar/.local/bin/pomo

@@ -0,0 +1 @@
+../share/pomo/pomo.sh