Simplify format options on the CLI

[mwouts]

Jupytext can be used to set format options on the command line (#130) with e.g.

jupytext --opt split_at_heading=true notebook.Rmd

[codecov]

Codecov Report

Merging #503 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #503   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files          83       83           
  Lines        8098     8110   +12     
=======================================
+ Hits         7997     8009   +12     
  Misses        101      101           

Impacted Files	Coverage Δ
jupytext/cli.py	95.45% <100.00%> (+0.01%)	⬆️
jupytext/paired_paths.py	97.50% <100.00%> (ø)
tests/test_cli.py	99.86% <100.00%> (+<0.01%)	⬆️
tests/test_read_simple_markdown.py	100.00% <100.00%> (ø)

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