Autodetect pip installation on windows

[lorenzwalthert]

Working towards #128.

[codecov-io]

Codecov Report

Merging #133 into master will decrease coverage by 0.86%.
The diff coverage is 21.42%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   74.35%   73.48%   -0.87%     
==========================================
  Files           8        8              
  Lines         308      313       +5     
==========================================
+ Hits          229      230       +1     
- Misses         79       83       +4     

Impacted Files	Coverage Δ
R/exec.R	45.83% <21.42%> (-3.01%)	⬇️

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