Jupytext extensions should be aware of the default Jupytext configuration

[mwouts]

In 1.0.0-rc4 the Jupytext extensions are completely blind to the default Jupytext config. We should change this and at least implement @phaustin's expectations #176 :

I expected the "custom" item to be checked for me in the pull down menu, since I've got c.ContentsManager.default_jupytext_formats = "ipynb,python//py:percent