CLI11 Branding

[henryiii]

Adding support for making custom versions of CLI11 single header. This way, you can implement CLI11 into a custom project by adding a custom namespace around CLI, as well as replace the headers. For example, making a custom include for ROOT would look like:

./scripts/MakeSingleHeader.py ROOT_CLI.h --namespace ROOT --macro CLI11_ ROOT_CLI_ 

[codecov]

Codecov Report

Merging #150 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #150   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1691   1691           
=====================================
  Hits         1691   1691

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