Add diff functionality to mount

This is a proposal for a new function, that does the following:
Add an option to the mount command to mount only files/directories that changed in between two archives.
It would be even more awesome, if files could be further filtered by the categories given by the diff command (i.e. added, removed, ...). For the filter command itself this could also be viable, but less so, as it can already be done by grepping its output.

Why do I want this: I use the following structure with rsync based backups to quickly find/retrieve changes on different platforms (i.e. nfs/smb mounted):

archive/

  full/

  incr/ (only changes to previous backup)

  del/ (only files deleted since previous backup)

    dates/

       backup/ (actual original directory structure)

I am aware of the new merged mount option in 1.1.0, which will allow to quickly visualize changes, but not really restore - so these additions are still worthwhile. Actually they would play together perfectly.

Thanks once more for all your work - borg is awesome!