Complete bug IDs, bridges, users, labels where appropriate.

This works in bash and fish. ZSH is not yet supported by Cobra.
In fish, descriptions (the part of a completion after the "\t") are shown
as completion label, and can be searched with Ctrl+S.

Reproduce with

	fish -C 'source misc/fish_completion/git-bug'
	git bug select ^I

(tested with fish version 3.3.1)

Also works with bash, but only for "git-bug" (with the dash)

	bash --rcfile <(echo source misc/bash_completion/git-bug)
	git-bug select ^I

Closes git-bug#493