zsh completions

This pull request adds new command line option `-d/--cheat_directories` and zsh completion file.

With this option `cheats` prints a list of directories containing cheatsheets.
It is used in zsh completion file to list all available cheatsheets.

Zsh completion file is almost the same as in https://github.com/chrisallenlane/cheat/pull/20
In future I can write a more complete completions with all cheats options support