-o/--override-ini: add support for appending (`foo+=bar`)

[blueyed]

Seems to be better than a new pair of options, especially with regard to / when using a short letter for it.
(Upstream refs: pytest-dev#5685, pytest-dev#7391)

[gnikonorov]

Wouldn’t this just be a dupe of pytest-dev#5685 but with a different flag?