allow specifying the set of attributes that should be used to configure the configurable product

We ran into a problem where a customer had an attribute set with three optional, configurable attributes. They wanted products in this attribute set to *sometimes* use just one or two of those configurable attributes to configure their configurable products, while leaving the others out. This plugin as written assumed that *all* configurable attributes in the set would *always* be used to configure the configurable product. This change adds an optional `configurable_attributes` element to the request, that can contain a list of attributes, by ID, that should be used to configure the configurable product.