updates to nbextensions_configurator: * don't use a mutable value (list) as a default argument * updates to docstrings
updates to nbextensions_configurator:
* don't use a mutable value (list) as a default argument
* updates to docstrings

original-commit-id: bdd0388e37b4ef989a0a7cc061642faab0fe9aa1
original-commit-url: https://github.com/ipython-contrib/IPython-notebook-extensions/commit/bdd0388e37b4ef989a0a7cc061642faab0fe9aa1

jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator