Allow LQML extension function names to be URIs or namespace/localname pairs.

[clange]

For better extensibility it should be possible to have LQML extension functions in multiple namespaces. We say that we are inspired by http://www.w3.org/TR/sparql11-query/#extensionFunctions http://www.w3.org/TR/xpath-30/ – there, function names are URIs (SPARQL) or namespace/localname pairs (XPath). This would require a prefix-binding syntax in LQML, so that functions could be invoked as prefix:functionName. I'd give this a low priority.