Add PassUnknownFlagsToArgs to allow get unknown flags via Args()

[ikedam]

Close #337

FlagSet.ParseErrorsWhitelist.UnknownFlags=true behaves like this:

• Input:

  -xayb -c -z --known-flag known-flag-value --unknown-flag arg0 arg1 arg2
  

  • -x, -y, -z and --unknown-flag are unknown
• Args():

  arg1 arg2
  

This request adds FlagSet.ParseErrorsWhitelist.PassUnknownFlagsToArgs, and setting it to true results following behavior:

• Args() with FlagSet.ParseErrorsWhitelist.PassUnknownFlagsToArgs=true:

  -xy -z --unknown-flag arg0 arg1 arg2
  

Any unknown and unprocessed flags are passed to Args().

[CLAassistant]

All committers have signed the CLA.

[jippi]

@spf13 👋 Any chance of this landing? I have exactly this use-case as well :)

If it needs work, I would be happy to take it on