Bugfix: fix optional positional arguments.

[joshlk]

Optional positional argument previously raised an error with argparse:

TypeError: 'required' is an invalid argument for positionals

Fixes #346

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.29%. Comparing base (a70c4c7) to head (e814070).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   86.29%   86.29%           
=======================================
  Files          34       34           
  Lines        4553     4554    +1     
=======================================
+ Hits         3929     3930    +1     
  Misses        624      624           

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[lebrice]

Awesome, neat pr @joshlk , thanks a lot!