Add option to backup recent images only (with a custom cut-off date)

[t-karcher]

When backing up the same Wiki on a regular basis, saving all images in every dump is not only time-consuming, but also a waste of space, as images rarely change after the initial upload. It would be nice if the dumpgenerator had the option to only backup images added after a given date, to speed up the backup process and save some space.

[robkam]

Also requested at #51