[Feature] support --recursive in combination with a folder as an argument

[bneijt]

I would like support for recursive to have all the notebooks under a given folder to be considered for cleaning.

nb-clean clean --recursive banana.ipynb src would clean both banana.ipynb and all *.ipynb below the folder src.

This relates to issue #140 because that could also be achieved using this option.

[bneijt]

During PR discussion the feature was changed to not use a commandline argument but recurse by default for any given path that is a directory.

[srstevenson]

Implemented in #148.