Add getFilterArgs method in DataView

Useful for combine filters easily. Example:

        $(grid.getHeaderRow()).on("change keyup", ":input", function(e) {
            var $this = $(this),
                columnFilters = {};

            columnFilters[$this.data("columnId")] = $.trim($this.val());
            dataView.setFilterArgs($.extend({}, dataView.getFilterArgs(), columnFilters));

            dataView.refresh();
        });