Completions for common shells

[lgarron]

Please check everything that applies to your issue:

• I looked in closed issues and it has not already been answered
• My issue appeared with a terminal-notifier update
• I'm using a tool that uses terminal-notifier (guard-notifier, node-notifier...)

It's now becoming common for tools to ship with completions as a command, and Homebrew makes it easy to install these completions automatically.

It would be really nice to have completions printed by the following commands:

terminal-notifier -completions fish
terminal-notifier -completions zsh
terminal-notifier -completions bash

I could possibly help with the fish completions if they have to be manually written, but I don't have time right now.