Add PostMeta arg for "label"

It is used for human readable labels for Block Binding sources. Available in https://github.com/johnbillion/args/commit/d971d1c977f7bd1b89d17376a87a66e6fb5f5d49#diff-ec5ed3417512430b9c6ad953315a9aaea4f32cc5c3dc7725a74a648e3850ab2b.

To be done in https://github.com/wearerequired/common-php/pull/7.