Add -d/--cheat_directories option to list directories containing cheatsheets
Add zsh cheats completions
Install zsh completions
Merge branch 'zsh_completions' of https://github.com/a-sk/cheat into a-sk-zsh_completions

* 'zsh_completions' of https://github.com/a-sk/cheat:
  Install zsh completions
  Add zsh cheats completions
  Add -d/--cheat_directories option to list directories containing cheatsheets
Minor modifications on the cheat file. Broke the -d option out into its own logical block.