Feature request: Permit use of placeholders for custom output filenames

[entodoays]

It would be useful if there is the possibility that the output files be named according to a template. Some examples:
%y-%m-%d would translate to the current date.
%yorig-%morig-%dorig would refer to the original image's date
'#n could be a sequential number.
This would be very useful to me and I'm sure to others too.