Support customPattern as an alias for customDatePattern and customDateTimePattern

[joelittlejohn]

The extension properties customDatePattern and customDateTimePattern don't need to qualify themselves with the type name, since they live inside a the schema for the field already. It would be nicer for this tool to accept customPattern in a variety of places. We can support this in more places as time goes by and there will be no need for users to learn more and more custom rule names.