Add .r extension for R files

[kiendang]

Allow .r which is a valid extension for R files.

[codecov]

Codecov Report

Merging #122 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files          52       52           
  Lines        3737     3737           
=======================================
  Hits         3631     3631           
  Misses        106      106

Impacted Files	Coverage Δ
jupytext/formats.py	96.89% <ø> (ø)	⬆️
jupytext/languages.py	100% <ø> (ø)	⬆️

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[mwouts]

Thanks @kiendang for the contribution - I agree that Jupytext should allow '.r' extension as well. I am a bit surprised that the change can be so small! Does it work well with only this change?

A possible list of tests would be

• open .r file as notebooks in Jupyter
• export to .r files using "jupytext": {"formats": "ipynb,r"}
• convert .ipynb files from and to .r files using command line jupytext.

[kiendang]

Yea sorry I only tested the first point. I have added a new commit that takes care of point 2 and 3.

[mwouts]

Great! I will have a look at this soon. Thanks Dang.

[mwouts]

Integrated in version 0.8.5. Thanks @kiendang !