Add support for C# and F#

[mwouts]

This provides support in Jupytext for C# and F# notebooks, cf. #427

[codecov]

Codecov Report

Merging #429 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
+ Coverage   98.94%   98.98%   +0.04%     
==========================================
  Files          75       77       +2     
  Lines        7645     7705      +60     
==========================================
+ Hits         7564     7627      +63     
+ Misses         81       78       -3

Impacted Files	Coverage Δ
jupytext/metadata_filter.py	100% <ø> (ø)	⬆️
tests/test_read_dotnet_try_markdown.py	100% <100%> (ø)
tests/test_read_simple_csharp.py	100% <100%> (ø)
tests/test_active_cells.py	100% <100%> (ø)	⬆️
jupytext/version.py	100% <100%> (ø)	⬆️
jupytext/cell_metadata.py	98.29% <100%> (+1.61%)	⬆️
jupytext/magics.py	100% <100%> (ø)	⬆️
jupytext/jupytext.py	98.59% <100%> (-0.01%)	⬇️
jupytext/languages.py	100% <100%> (ø)	⬆️
jupytext/formats.py	98.35% <100%> (-0.32%)	⬇️
... and 37 more

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[cartermp]

Two other file extensions for F#:

• .fsx - F# scripting file, can be loaded directly into the F# Interactive process
• .fsi - F# interface file (similar to OCAML interface files), used mostly in very large codebases to encapsulate code constructs

.fsi is probably not relevant to this project, but .fsx might be.

[mwouts]

I see that C# uses specific magic commands. See e.g. this notebook, which uses for instance:

#!html

<b>Hello!</b>