Hide notebook metadata

[mwouts]

This PR introduces the hide_notebook_metadata option for the Markdown format.
Closes #527 .

[codecov]

Codecov Report

Merging #581 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
+ Coverage   98.98%   99.00%   +0.01%     
==========================================
  Files          89       89              
  Lines        8794     8841      +47     
==========================================
+ Hits         8705     8753      +48     
+ Misses         89       88       -1     

Impacted Files	Coverage Δ
jupytext/formats.py	97.20% <ø> (+0.31%)	⬆️
jupytext/cli.py	96.48% <100.00%> (+0.01%)	⬆️
jupytext/config.py	96.24% <100.00%> (+0.08%)	⬆️
jupytext/header.py	99.31% <100.00%> (+0.07%)	⬆️
tests/test_cli_config.py	100.00% <100.00%> (ø)
tests/test_header.py	100.00% <100.00%> (ø)
tests/test_read_simple_markdown.py	100.00% <100.00%> (ø)

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