need output-method json

[philshafer]

(Originally http://code.google.com/p/libslax/issues/detail?id=10 )

When dealing with generating json, a script can use "output-method text;"
and use xutil:xml-to-json() to translate the output XML to JSON.

I'd like to make this a first-class output method. It would translate into <xsl:output method="text"/> but would turn on a flag that would call the internal version of xml-to-json() when the script is done.