add ability to find notebooks recursively

[hamelsmu]

Closes #358

Note: I made sure that .ipynb_checkpoints is ignored by ignoring all paths that contain the substring /., which ignores all hidden directories.

Ready for review: @jph00

This also partially related to #357.

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[jph00]

This should be optional and default to false, since otherwise it's a breaking change

[hamelsmu]

ok @jph00 ready for review. I made the following changes

• made recursive search optional
• updated the nbdev_build_lib CLI interface (I forgot to do that previously for some reason)
• moved the notebook glob search to its own function
• Updated tests in showdoc that incidentally use this function to test something else about showing the signature

cc: @muellerzr

[hamelsmu]

@jph00 made changes you requested