This adds kParseTrailingCommasFlag to allow a trailing comma at the
end of maps and arrays. This is part of issue #36, adding optional
support for relaxed JSON syntax.

Fixed up some wording to the schema documentation

#499

Credits to @pah who suggested it in comment to #368.

Closes #366

Closes #368

AddMember requires allocator, this fix keeps all uses of AddMember consistent across the whole tutorial.