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.