add renderSelectedChoices style option

[matthyk]

Closes #1349 and supersedes #1350

I decided not to pass the current theme to the function. I think the function should really only determine how the selected options should be rendered. Further customisation can already be done with the answer style option.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44016a4) 94.47% compared to head (0e270fa) 94.48%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1360   +/-   ##
=======================================
  Coverage   94.47%   94.48%           
=======================================
  Files          53       53           
  Lines        4558     4566    +8     
  Branches      777      778    +1     
=======================================
+ Hits         4306     4314    +8     
  Misses        247      247           
  Partials        5        5           

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[SBoudrias]

Thanks for following-up! LGTM, just left a nit and we'll be good to merge/release.