Custom namespaces for XPath #621

I created a sample document in Word and the namespaces contained within it seem to be different than those that are natively included with python-docx. I found the included namespace map here.

And it appears that the newest version overrides the XPath function and uses these included namespaces. In previous versions of python-docx I would use the child's namespace directly in the function to ensure that it was found, but with this override I am unable to do it and get the error:

TypeError: xpath() got an unexpected keyword argument 'namespaces'

Here is my sample document:

https://github.com/fdabek1/random_files/blob/master/checkbox_control.docx

Below is some sample code:

from docx import Document

doc = Document('checkbox_control.docx')

for paragraph in doc.paragraphs:
    checkboxes = paragraph._element.xpath('w14:checkBox')

    # Old method that no longer works
    # checkboxes = paragraph._element.xpath('w14:checkBox', namespaces=paragraph._element.nsmap)

Being able to get this functionality back would be great as I am using python-docx to parse through some Word documents with a lot more ease than using lxml/etc for the entire document.