Provide a better way to override allow-duplicate-labels

[ceremcem]

If a footprint is created by overriding create() method, data.allow-duplicate-labels can not be set easily. Here is current way:

provides class DG142V_5_08_XXP extends Footprint
    (data, overrides) ->
        super data, overrides `based-on` {
            +allow-duplicate-labels
        }

    create: (data) ->
        ...

There should be a way to override this property within the create() method to make the footprint definition more human readable.