Read config from `setup.cfg`

[janosh]

Would be cool if config options could be specified in setup.cfg so that they apply both when called from CLI and via pre-commit.

# setup.cfg

[nbstripout]
drop_empty_cells = true

[kynan]

Very much agree! Feel like working on this @janosh ? :)

[kynan]

There's an unfinished draft PR for this in #169. Anyone interested in this feature, please pick up this PR :)