cargo upgrade: Suggest increasing project's version after Semver-major upgrades

[killercup]

cf. https://www.reddit.com/r/rust/comments/6p8trs/cargoedit_which_contains_cargoaddrmupgrade_just/dkolga3/

Should probably happen after #143

[leoyvens]

I guess whether to suggest this or not depends on the public/private dependency distinction.

[epage]

Blocked on rust-lang/rust#44663

[epage]

I'm assuming cargo upgrade will merge before public dependencies are done. I've transfered this to the task list in #685.