Add path exists validator

[lczech]

Fixes issue #63

[lczech]

Oh. Just realized that this also contains a small other PR... Sorry for that, hope that's okay.

[codecov]

Codecov Report

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

@@          Coverage Diff          @@
##           master    #73   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1360   1358    -2     
=====================================
- Hits         1360   1358    -2

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

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