add gitignore, ignore custom.css and custom.js
add gitignore, ignore custom.css and custom.js

original-commit-id: d220e7b52a3ff982a63658560f8c3451244c7901
original-commit-url: https://github.com/ipython-contrib/IPython-notebook-extensions/commit/d220e7b52a3ff982a63658560f8c3451244c7901

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