Optional variables

[fuzaylov]

Please add support for optional variables. For example:

'variables' => [
    [
        'label' =>  'quantity',
        'field' => 'number',
        'required' => false, // indicates whether this is a required field OR
        'optional' => true, // indicates whether this is an optional field
    ],

[stepanenko3]

@Fuzaylov, hi. Unfortunately, I don't have time now. But I'm ready to accept PR