Allow to pair notebooks in trees rather than in folders

[mwouts]

That was suggested by @technic at #265:

I want to have to directory trees with the same structure, but one with the ipynb files, and another one with python files.

Maybe we could use, say jupytext.formats="/notebooks//ipynb,/scripts//py" (using e.g. the initial / to indicate that path is matched from the root rather than from the parent dir) to pair notebooks and scripts in trees rooted at /notebooks and /scripts. For example, /notebooks/foo/bar/qqq.ipynb would be paired with /scripts/foo/bar/qqq.py.

[mwouts]

I have implemented something like this in Jupytext 1.5.0 (RC coming soon). You'll have to use e.g.
jupytext.formats="notebooks///ipynb,scripts///py" (triple slash), and this gives the pairing in tree. See for instance this test:

jupytext/tests/test_cli.py

Lines 1018 to 1027 in ccb736c

	def test_pair_in_tree(tmpdir):
	nb_file = tmpdir.mkdir("notebooks").mkdir("subfolder").join("example.ipynb")
	py_file = tmpdir.mkdir("scripts").mkdir("subfolder").join("example.py")
	write(new_notebook(cells=[new_markdown_cell("A markdown cell")]), str(nb_file))
	jupytext(["--set-formats", "notebooks///ipynb,scripts///py:percent", str(nb_file)])
	assert py_file.exists()
	assert "A markdown cell" in py_file.read()

[mwouts]

The rc is available:

pip install jupytext==1.5.0rc1

Please give it a try, and let me know if it works as expected!