Add a simple data type for dates

[mbrush]

Can extend string, like iri and code data types do.

Consider if we want date or dateTime.

See proposals from Matt's and Bob's respective VA- and GA4GH- data type documents.

[larrybabb]

in json schema we are using the format attribute that is a sibling to the type: string declaration that allows us to control whether the particular property is a specific format of a date (time, date or datetime).

I suppose we could add something to the metaschema processor to maybe make the various data datatypes appear more cleanly, but I think we should defer that until there is a more urgent need.

If this is primarily about the spec and it's documentation we can develop a way to generate these date-based datatypes in the documentation.