Translate R Markdown visibility options

[mwouts]

R Markdown options for hiding the cell input/output, or the entire cell, are now mapped to either the Jupyter Book tags remove_input/remove_output/remove_cell (by default), or to the hide_input/hide_output metadata used by the runtools extension for Jupyter Notebook (when the option use_runtools is activated).

Closes #337

[codecov]

Codecov Report

Merging #366 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   99.07%   99.05%   -0.03%     
==========================================
  Files          73       74       +1     
  Lines        7262     7311      +49     
==========================================
+ Hits         7195     7242      +47     
- Misses         67       69       +2

Impacted Files	Coverage Δ
jupytext/cell_to_text.py	97.41% <100%> (ø)	⬆️
tests/test_read_simple_rmd.py	100% <100%> (ø)	⬆️
jupytext/formats.py	98.98% <100%> (ø)	⬆️
jupytext/jupytext.py	100% <100%> (ø)	⬆️
tests/test_active_cells.py	100% <100%> (ø)	⬆️
jupytext/cell_reader.py	96.91% <100%> (ø)	⬆️
tests/test_hide_remove_input_outputs_rmarkdown.py	100% <100%> (ø)
jupytext/cell_metadata.py	96.59% <100%> (-0.57%)	⬇️
tests/test_cell_metadata.py	100% <100%> (ø)	⬆️
... and 1 more

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