Support custom datatypes for literals in BindingModels.

[bnouwt]

I would like to be able to use custom datatypes in my binding models. For example, the following Python code creates a custom binding model with literals that have XSD datetypes.

class ObservationBinding(BindingModel):
    o: Uri
    em: Uri
    timestamp: Literal[datetime]
    result: Uri
    prop: Uri
    val: Literal[float]

However, sometimes (very rarely) you would like to be able to use custom datatypes in your binding model. Is this possible? Or can we use the knowledge-mapper without these BindingModels and just return a custom JSON binding set in those cases?