borg extract: only newer/non-existing files

## Have you checked borgbackup docs, FAQ, and open Github issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

Question

## Describe the problem you're observing.

Is there a way to have 'borg extract' ignore already existing/extracted files ? This would be useful if the connection is lost during an extract, or if halfway a large restore you decide it's better to abort and add an additional --exclude mask.