feat: Add Bash completion for Homebrew aliases

[yermulnik]

Add Bash completion support for Homebrew aliases:
https://github.com/Homebrew/homebrew-aliases

Unfortunately I'm not familiar enough neither with Ruby, nor with Fish/Zsh 🤷🏻

Fixes Homebrew/homebrew-aliases#91

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• Have you written new tests for your changes? Here's an example.
  • It somewhat exists: https://github.com/Homebrew/brew/blob/master/Library/Homebrew/test/completions_spec.rb#L303-L316
• Have you successfully run brew style with your changes locally?
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[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @yermulnik!