`repeat` options is not considered as an unknown option with `unknown-options-as-args`

[mleguen]

Initial issue: yargs/yargs#1489

const Parser = require('yargs-parser')

console.log(Parser(process.argv.slice(2), {
  configuration: {
    'unknown-options-as-args': true
  },
  string: ['known']
}))

Expected output:

$ node index.js --known x --repeat 100 --unknown 200
{ _: [ '--repeat', '100', '--unknown', '200' ], known: 'x' }

Actual output:

$ node index.js --known x --repeat 100 --unknown 200
{ _: [ '--unknown', '200' ], known: 'x', repeat: 100 }

No problem if using repear instead of repeat:

$ node index.js --known x --repear 100 --unknown 200
{ _: [ '--repear', '100', '--unknown', '200' ], known: 'x' }