Add support or mode to allow undefined namespaces

[aheiska]

I currently often find my self in situation where i have a part of larger xml-messages where namespace declarations are way higher in document. It would be helpful if I could take a child tag and format it.

Currently indenting following snippet fails:

<foo:task id="46ea7" active="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="foo:Task"><foo:source device="xyz" sourceId="xyxxy" /></foo:task>

Adding an xmlns:foo="aaa" to observed root of course makes it work, but this is manual work and has to be repeated for every namespace found in the snippet.