User-defined handling of repeated JSON object members.

src/json.m:
	Add a type that describes how the parser should handle repeated object
	members.

	Add field to the JSON reader parameters that specifies how to handle
	repeated object members.

src/json.json_parser.m:
	Handle the new parameter.

tests/test_json.m:
	Allow JSON reader parameters to be set on a test-specific basis.

tests/repeated_member_{first,last}.{json,exp}:
	Test the new behaviours.

README:
	Mention the above in the list of features.