Set output postfix for processing directories

[nicothed]

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First, thank you for an amazing package!

I'm trying to process an entire directory like this: deface "pics/" --replacewith mosaic --mosaicsize 20 --thres 0.4 --output "_mosaic20.jpg"

I tried different parameters and would like to use a different postfix for them (e.g. "mosaic20"). The issue is that the output path will be the same for all images (or I'm not understanding something). IMO, it would make a lot of sense to add a parameter "postfix" that simply adds a specified postfix to all outputs.