add zsh autocomplete for shovel tasks

Because shovel is awesome, but typing isn't. Because `help` does some pretty printing, I created a new method for the command line tool `tasks` that dumps out an alphabetical list of task `.fullname`s and a documentation snippet with simple, flat formatting like rake's `--tasks`. Then added a little zsh script.

Now using autocomplete in my repos and projects, and loving it.

Thank you for shovel!