extract: normalize tarball modes (like git)

[StefanKarpinski]

This would require a minor version bump since it changes the way extraction handles permissions: this changes it to ignore the exact permissions in the tarball and only pay attention to whether each file is user-executable and then behave as if the permissions where the git-normlized ones of 0o644 and 0o755. Unlike the other permission changes, which were bugfixes to deal with filemode being unreliable on some platforms, this is a policy change. It's currently a draft since I need to change the docs back to an older version where they claimed this was the behavior even though it actually wasn't at the time.

[codecov[bot]]

Codecov Report

Merging #99 (010f1a8) into master (61adcb8) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #99   +/-   ##
=======================================
  Coverage   96.36%   96.37%           
=======================================
  Files           4        4           
  Lines         633      634    +1     
=======================================
+ Hits          610      611    +1     
  Misses         23       23           

Impacted Files	Coverage Δ
src/extract.jl	96.26% <100.00%> (+0.01%)	:arrow_up:

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[StefanKarpinski]

Updated the docs so this is now ready for merging.