Add stale bot in this repo

[Alexandros]

The issue tracker of this repo is cluttered with inactive issues opened years ago.

So I will add stale to have these issues removed after a period of time.

Turns out that I cannot add stale since I am not an admin of this repo.

So @bep if you can please enable stale in this repo and then I will sort its settings and add a keep tag to issues that need to be kept active.

[Bjørn Erik Pedersen]

Done.