Nbconvert - clean old figures when repeating dump to markdown

HI,

When using nbconvert
jupyter nbconvert --to markdown --output-dir nbs_out --log-level WARN ...
A notebook may have its cells altered so the same figure appears twice but from different cells.
Is is possible to have nbconvert clean the figures directory first?
It looks like --clear-output will clean the notebook itself.

Deleting the directory will work but I would like a one-liner for my pre-commit.
Thanks,
Joe