Add a way to get the argument attempted to be overwritten from an OverwrittenOptionException

[acid1103]

Provides a nice way to get the ArgSpec of the option which was being overwritten before throwing an OverwrittenOptionException. The ArgSpec is retrieved through OverwrittenOptionException#getOverwritten(), which can be taken advantage of in a custom IExceptionHandler2 to automate exception handling, generate custom error messages, etc.

I was in the process of writing a custom IExceptionHandler2 and noticed some ParameterExceptions have convenience methods for accessing the root of the exception (see MissingParameterException#getMissing()) and thought OverwrittenOptionException was missing such a method. It's a tiny pr, but seems useful to me at least. Thanks for the consideration.

[codecov-io]

Codecov Report

Merging #532 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #532      +/-   ##
============================================
+ Coverage     89.14%   89.15%   +<.01%     
  Complexity      285      285              
============================================
  Files             4        4              
  Lines          3933     3936       +3     
  Branches        964      964              
============================================
+ Hits           3506     3509       +3     
  Misses          213      213              
  Partials        214      214

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	89.18% <100%> (ø)	160 <0> (ø)	⬇️

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[remkop]

Thanks for the pull request!