added unoconv call to convert wmf images from docx -- currently untested due to external bugs
replace wmf links in xml document with png links
Merge remote-tracking branch 'upstream/master'
refactored .wmf image conversion to properly use xpath
Further incorporation of wmf replacement(#31)

This now successfully replaces the file extension in the TEI XML.

- [x] replace wmf with png equivalent
- [ ] test cases for XML replace
- [ ] call to LibreOffice/ImageMagick to convert the file (waiting on upstream?)