Allow user-specified backup directory format.

[arensb]

The dirvish-report.sh script doesn't support the dirvish image-default option: it just assumes that backup directories are named in the format %Y%m%d.
I've added support for single-line options in
arensb@0036bd9
and added the check for image-default in
arensb@b81597a

[MarvAmBass]

Hi Andrew, thanks for you work! I really appreciate it!

Did you test everything? are there any breaking changes?