Update CommandOption/Argument.value to return DefaultValue

[natemcmaster]

Follow-up to #389. This updates CommandOption.Value and CommandArgument.Value to return DefaultValue when nothing has been passed on command line.

[codecov]

Codecov Report

Merging #420 (8cd2a2a) into feature/default-value (9ff8c22) will increase coverage by 0.58%.
The diff coverage is 100.00%.

@@                    Coverage Diff                    @@
##           feature/default-value     #420      +/-   ##
=========================================================
+ Coverage                  67.17%   67.76%   +0.58%     
=========================================================
  Files                        105      105              
  Lines                       3260     3282      +22     
=========================================================
+ Hits                        2190     2224      +34     
+ Misses                      1070     1058      -12     

Impacted Files	Coverage Δ
src/CommandLineUtils/CommandArgument.cs	94.73% <100.00%> (+6.50%)	⬆️
src/CommandLineUtils/CommandArgument{T}.cs	96.66% <100.00%> (+25.23%)	⬆️
src/CommandLineUtils/CommandOption.cs	69.01% <100.00%> (+0.89%)	⬆️
src/CommandLineUtils/CommandOption{T}.cs	96.66% <100.00%> (+25.23%)	⬆️
.../CommandLineUtils/Internal/CommandLineProcessor.cs	83.04% <0.00%> (+0.43%)	⬆️

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