Add common config section for frontend config

This is a place to store config that should be used by all pages (notebook, dashboard, editor, terminal).

The first such thing is nbextensions that apply to all pages. Ping @Carreau , because this is probably relevant for the Google Drive extension.
-1 on using IPython global, as load_extension is defined in same module. 

if you fix it could you remove as the same time `IPython.load_extension` and replace it with a `var load_extension`at the top of this file, export it and bind it to IPython later?