Add ability to skip comma flag even if no comma output implemented

[simonhf]

Addresses half of issue #71 by adding ability to ignore a comma flag -- e.g. #'d -- and not interpret it literally. The result is as if the comma flag is not present, e.g. #d. All existing tests work, and added a new test section for the comma flag.

This pull request at least makes this implementation of printf() compatible with existing code which uses the comma flag... At least seg faults and/or unwanted output no longer occur. Next step will be to implement the comma output with some kind of mechanism to turn it on; environment variable?

[codecov-io]

Codecov Report

Merging #73 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #73   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         359    360    +1     
=====================================
+ Hits          359    360    +1

Impacted Files	Coverage Δ
printf.c	100% <100%> (ø)	⬆️

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