Add watched function to keep track of shows

Usage:
* watched (no arguments): lists the current items being watched
* watched "show": will advance the show episode and timestamp the entry
between the brackets: <>
* unwatch "show": will decrement the number of the show

The watch.txt file in $TODO_DIR that has this format:

 show <> 1
 another show <> 1
 yet another show <> 1