Optional choices for array

I have a config like this:

filetype: {
  choices: ['.js', '.jsx'],
  type: 'array'
},
recursive: {
  type: 'boolean'
}, 

So this works:

create-index --filetype .js .jsx --recursive ./src

But this throws an error:

create-index --filetype .js .jsx ./src
Invalid values:
  Argument: filetype, Given: "./src", Choices: ".js", ".jsx"

There should be an option to make the choices be "optional" rather than throw an error. If it doesn't find a match it should consider the rest of the arguments to be positional arguments. This can be especially useful for arrays as shown in above example. It should throw (in case of an array) only if the first argument doesn't match.

This could possibly address yargs/yargs-parser#219

Reproduce the two create-index command examples and compare how array choices handle the following positional argument. Review the related yargs/yargs-parser#219 discussion and the parser's existing choice and positional-argument tests; done means an optional choice can leave unmatched trailing arguments positional while still rejecting an unmatched first array value.