Making add_help optional for parse and parse_known_args functions

[farzad-elastix]

This PR is very limited in scope. It's probably easier to just read the 4 LoC changed. The default is the right one most of the time but I've run into an issue where I need to handle the CLI args in multiple steps and that means I'll have to disable help in the first stages until I get to the final stage.

The changes are non-destructive and do not change the default behavior.

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I'll take the opportunity to say my thank for the repo. I love the simplicity and how it promotes better coding practices over the original argparse. I've run into the rough edges (e.g. handling nested items or dictionaries), but still recommend it.

[farzad-elastix]

cc @lebrice: deciding on this fix should be very quick whenever you have the time.

[lebrice]

Hey @farzad-elastix , thanks a lot! I'll take a look.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.29%. Comparing base (2a69ceb) to head (b87a577).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #360   +/-   ##
=======================================
  Coverage   86.29%   86.29%           
=======================================
  Files          34       34           
  Lines        4554     4554           
=======================================
  Hits         3930     3930           
  Misses        624      624           

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