New argument --run-path

[mwouts]

Closes #595

[codecov]

Codecov Report

Merging #612 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   98.99%   98.98%   -0.01%     
==========================================
  Files          94       94              
  Lines        9152     9184      +32     
==========================================
+ Hits         9060     9091      +31     
- Misses         92       93       +1     

Impacted Files	Coverage Δ
jupytext/cli.py	96.96% <100.00%> (+0.11%)	⬆️
tests/test_execute.py	100.00% <100.00%> (ø)
jupytext/formats.py	96.60% <0.00%> (-0.31%)	⬇️

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