`extract` to optionally follow symlinks

Following symlink is useful in many use-cases.
One of them is during a partial extract.
Let's say mysqldump are stored somewhere, and the last one is linked to "latest-dump.sql.gz"
Extracting only such a dump from a backup is made much more easy if symlinks are followed.

Eg: could be a `-L, --dereference` option to `borg extract`.
I can't provide a PR, but at least for now, I wanted to suggest/discuss this feature.