Per-diagnostic warning/error policy

[kyrias]

We're using mdformat for our documentation which has now started removing the escapes from regular square bracketed text, with the rationale that under the CommonMark spec they should be equivalent and so the escaping is unnecessary. This means that mdbook-linkcheck now warns about all of these being potentially incomplete links.

While it's possible to set warning-policy to ignore what we want is to be able to only disable the incomplete link warning while leaving the absolute link warning, and any warnings that might be added in the future, in effect.

So it would be useful if there was a system for setting the policy per-diagnostic instead.