Support for csvw spec 'separator'

[SimonGreenhill]

csvw can define 'separators' in field definitions e.g.

                    {
                        "datatype": "string",
                        "propertyUrl": "http://cldf.clld.org/v1.0/terms.rdf#source",
                        "required": false,
                        "separator": ";",
                        "name": "Source"
                    }

...which means that the field should be parsed from "a;b" to something like c("a", "b"). It would be nice to support this.