Allow to deny multiple-versions except for a list of crates

[Mike Hommey]

While

[bans]
multiple-versions = "deny"

is very much desirable, it is hard to fulfill completely. The typical example right now would be syn, where many crates are still using version 1.0.

On the other hand, while there is a documented way to deny multiple-versions of a specific crate, there doesn't seem to be the opposite : denying them all except a given crate.

[Jake Shadle]

https://embarkstudios.github.io/cargo-deny/checks/bans/cfg.html#the-skip-field-optional

[Mike Hommey]

May I suggest an adjustment to the doc to use the word "multiple", like everything else related to the feature? (likewise for the skip-tree feature).

[Mike Hommey]

Actually the expression "duplicated versions" seems out of place. It should be either "duplicated crates" or "multiple versions".