Added a flurry of improvements to the brew fish completions

[lwolfsonkin]

• Add --language option to brew install and brew audit completion
• Add brew cask upgrade --dry-run completion
• Add cask completions to brew cleanup
• Drop completions for now-deleted brew prune command

Also, fixed documentation on website to reflect that brew ships fish completions now.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[MikeMcQuaid]

Thanks again @ynppny!