This issue is a follow-up of the discusion with @bcoe and @gajus in #1412

Example with command handlers:

• at the moment:
  • we accept async handlers for commands
  • we asynchronously log an error in parse() and showHelp() (and maybe other functions as well?) when an async handler reject
  • as parse() and showHelp() are sync, the calling program has no way to know if and when such an error will occur
• expected behavior:
  • when an async handler needs to be called, the calling Yargs function (parse(), showHelp(), etc.) should return a promise, to be resolved/rejected after the handler completes, for the calling program to know what is happening
  • when the handler is sync, the current behavior should be kept, ie the calling Yargs function should not return a promise

Before opening any PR, please detail in this issue the intended conception for review, and do not limit it to the command handler example detailed here (there may be other cases of async input).