feat: allow not to flatten duplicate array types

[laggingreflex]

Lets you disable flattening of multiple array type values

yargsParser('-x a b -x c d', {array: ['x]})
// => {x: ['a', 'b', 'c', 'd']} // all flattened

yargsParser('-x a b -x c d', {array: ['x], configuration:{'flatten-duplicate-arrays': false}})
// => {x: [['a', 'b'], ['c', 'd']]} // multiple nested arrays

yargsParser('-x a b', {array: ['x], configuration:{'flatten-duplicate-arrays': false}})
// => {x: ['a', 'b']} // still a flat single array when no duplicates

fixes yargs#688

includes #67

[bcoe]

@laggingreflex thanks for the great contributions 👍