Extended content nodes in writer

[ianprime0509]

It would be nice to have an API to write a single node's content to an XML document in a streaming manner. For text nodes, this is easy, as a simple std.io.AnyWriter wrapper can be written around the text function (though care must be taken to ensure this is done in a way that makes sense for XML raw and generic writers alike). However, for CDATA, comments, and PIs, this requires keeping track of additional state to know that the writer is inside an already open node (and close it when appropriate).