Use autocompletion when asking files from the user

[MisterDA]

Sometimes dune-release asks interactively from a path from the user (for instance, when asking the path to the cloned opam-repository). It would be nice if dune-release was using some sort of readline-like functionality to provide tab-autocompletion of the path.

[NathanReb]

That'd be good indeed!

If you feel like contributing I'm happy to guide you through it!