Add a ! operator to Validators for checking a false condition

[phlptp]

This PR is the first of a series pulled from #229.
If merged this PR will add support for a ! operator on Validators.

For example

app.add_option("--file", filename)->check(!CLI::ExistingFile);

[codecov]

Codecov Report

Merging #230 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #230   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        2057   2072   +15     
=====================================
+ Hits         2057   2072   +15

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

---

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[codecov]

Codecov Report

Merging #230 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #230   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        2057   2102   +45     
=====================================
+ Hits         2057   2102   +45

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <ø> (ø)	⬆️
include/CLI/Validators.hpp	100% <100%> (ø)	⬆️
include/CLI/Option.hpp	100% <0%> (ø)	⬆️
include/CLI/StringTools.hpp	100% <0%> (ø)	⬆️
include/CLI/TypeTools.hpp	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5e0bb1c...04a9c9d. Read the comment docs.