upgrade: Add short option -I for --ignore-rust-version

[mbuesch]

This PR adds a short option -I for upgrade's --ignore-rust-version.

This is an option which I use frequently - basically every time.

The idea is to first upgrade dependencies to the rust-version compatible crates, then commit that change, and then upgrade again with --ignore-rust-version enabled just to check if it would make sense to bump rust-version for the project.

Therefore, this option is a pretty frequent option for my use case and I think it deserves a short version.

[epage]

If cargo doesn't have it, we likely won't. Anything else should likely be worked out in an Issue rather than a PR. To better understand why, see https://epage.github.io/dev/pr-style/#d-issue

I should move this repo to my template which lists this in the contrib guide.

[mbuesch]

Ok. I'll be adding a local shell alias then.