Fixing #168

[henryiii]

This isn't my favorite solution, but it fixes #168 for now. The "forward" parsing bit (filling in prog --help sub so that it prints sub's help command) might not work well with custom callbacks, etc, since it runs the whole subcommand parse before continuing on.

A solution for 1.7.0 might be to break up _parse further into its components, then call them a bit more carefully.

[codecov]

Codecov Report

Merging #169 into master will decrease coverage by 0.05%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage     100%   99.94%   -0.06%     
==========================================
  Files          12       12              
  Lines        1699     1712      +13     
==========================================
+ Hits         1699     1711      +12     
- Misses          0        1       +1

Impacted Files	Coverage Δ
include/CLI/App.hpp	99.86% <92.3%> (-0.14%)	⬇️

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