cxml: add attribute namespace detection
cxml: add attribute namespace detection

So for example, an ‘r:id’ attribute causes an xmlns:r= namespace
declaration to be added to the element.

scanny/python-pptx,hoopes/python-pptx,biggihs/python-pptx,cchanrhiza/python-pptx,AlexMooney/python-pptx,kevingu1003/python-pptx