Use config in read and write functions

[mwouts]

This PR adds a config argument to the jupytext.read(s) and jupytext.write(s) functions.
This should help fixing #752

What remains to be done:

• The config is used only when saving (except for the rst2md conversion which has to be used when reading the file) that is not a good idea - the notebooks with no metadata don't store the format options, so for these notebook at least it makes sense to use the config also when reading them.
• We should deprecate default_metadata_filter in favor of metadata_filter, etc, and also default_jupytext_formats in favor of formats (and provide backward compatibility) => The PR for that is at Normalize jupytext config options names #754

[codecov]

Codecov Report

Merging #753 (4a474ea) into master (593bcef) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #753   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         106      106           
  Lines        9939     9948    +9     
=======================================
+ Hits         9850     9859    +9     
  Misses         89       89           

Impacted Files	Coverage Δ
tests/test_metadata_filters_from_config.py	100.00% <ø> (ø)
jupytext/cli.py	96.13% <100.00%> (-0.07%)	⬇️
jupytext/combine.py	100.00% <100.00%> (ø)
jupytext/config.py	97.48% <100.00%> (-0.07%)	⬇️
jupytext/contentsmanager.py	97.82% <100.00%> (-0.02%)	⬇️
jupytext/header.py	99.35% <100.00%> (+<0.01%)	⬆️
jupytext/jupytext.py	98.38% <100.00%> (+0.06%)	⬆️
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_contentsmanager.py	100.00% <100.00%> (ø)
tests/test_header.py	100.00% <100.00%> (ø)
... and 1 more

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