convert XML to/from JSON

[Francois Falala-Sechet]

Is your feature request related to a problem? Please describe.
Make it possible with pure CSML to convert JSON to XML and XML to JSON, for use with certain APIs

Describe the solution you'd like

XML2JSON("xml") => "json"
JSON2XML("json") => "xml"

Or maybe something more generic (and usable also with #192 )

Convert("xmlstring", from="xml", to="json")