Added optional error_handler to json_filter constructors
default_error_handler<Char> returns static instance of default error_handler, used in json_reader, json_filter, and csv_reader.
null_json_output_handler<Char> returns static instance of null null_json_output_handler, used in json_filter.
null_json_input_handler<Char>() returns static null json_input_handler instance
Cleanup
Updates to docs