streaming writer with extension API

[garthk]

I have to stream XML through something that:

• finds certain nodes
• notes their text
• finds certain following nodes
• fetch a URL based on the text noted above
• insert the results of fetching the URL as the text of a new child node

The original text is small enough to handle with ET.

If write could take a stream, and _serialize_xml could delegate the job of figuring out what to write to custom tag functions handed to Element, and the handler got the stream as an argument, I figure this could be pretty easy:

• fetch the URL
• match the encoding
• pipe the results into the outputstream without closing it at finish
• call back

As is, I figure I'll have to rip out ET and drop to a more conventional streaming parser and streaming writer.