Implemented basic XML reading capabilities
* command "read-xml-file" reads a XML file and allows it to be transformed by the calling XSLT
* this will allow workflows like the following:
** a tranformation reads all the markdown files in a directory
** these markdown files are converted to a xml representation by a external function calling a appropriate C++ markdown parser
** the XML representation of each markdown file is converted to XHTML inside the XSL transformation
** ... and embedded into the output XHTML document
** => all inside a single XSL tranformation
*** i.e. it is provided with only a empty dummy XML input file and fetches the actual input by itself
* as all current code contained within this repository this is just a quick and dirty proof-of-concept and not in any way good code