The fish completion hadn't been updated since 2009; this brings it up
to date. Changes include:
* Create the list of commands the same way as in bash completion
* Add every option for every current command, with descriptions
* Fixes a bug in fish_complete_brew_command
* Allow fish_complete_brew_command to take multiple commands
* Expand functionality of fish_complete_homebrew_formula to support
other commands and taps, and change name to reflect new functionality
Tested on fishfish, but should be compatible with fish.
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