Add single string parsing

[phlptp]

If merged this pull request will add a function to parse a single string containing all the arguments and optionally the program name. The function splits the single string into a vector and then uses the existing parse function. Basically a wrapper around the split function to deal with the program name and some edge cases that can arise when everything is in the same string.

The PR also includes test code for the functionality and
a modification of the split_up function to handle backquote '`' characters and quoted strings

[phlptp]

I didn't see any place in the readme where it would be appropriate to add a discussion of the function. So I didn't do anything regarding that or in the What's new file. I can add something if you would like.

[codecov]

Codecov Report

Merging #186 into master will decrease coverage by 0.05%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
- Coverage     100%   99.94%   -0.06%     
==========================================
  Files          12       12              
  Lines        1818     1841      +23     
==========================================
+ Hits         1818     1840      +22     
- Misses          0        1       +1

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/StringTools.hpp	99.12% <80%> (-0.88%)	⬇️

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[codecov]

Codecov Report

Merging #186 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #186   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1818   1849   +31     
=====================================
+ Hits         1818   1849   +31

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/StringTools.hpp	100% <100%> (ø)	⬆️

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[henryiii]

I have a few minor consistency changes in mind, but I'll go ahead and merge, then make them. I'll add at least a note in the change log.

[henryiii]

One suggestion: Most of the contents of the parse(string function can be moved to a helper function that is not part of App. This makes it easier to write unit tests for, since you have string in, vector of strings out, and keeps the already long App.hpp from growing too much.