Add extension allowing literal values to be specified

[lkitching]

The valueUrl property within column definitions allow URL values to be created, but other data types are not supported. Consider adding a column property for literal values e.g.

{"aboutUrl": "http://subject",
 "propertyUrl": "http://property",
 "value": "true",
 "datatype": "xsd:boolean",
 "virtual": true}