Add alias flag "--case-insensitive" for "--ignore-case"

[delvh]

Whenever I want to perform a case-insensitive search, I first wonder why there is a --case-sensitive flag, but no accompanying --case-insensitive flag when requesting command completion. Followed by that is a quick look in the man pages to find that the flag is called --ignore-case in reality. It would be nice if there were a convenience flag --case-insensitive that behaves exactly like --ignore-case.