[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/.local/share/pomo

@@ -0,0 +1 @@
+Subproject commit 68a8e26a46f8516ee73b429c153a1fc7a4a2c300