Docx reader: Make docx reader put image data in MediaBag.

Image data will not be put in a media bag map, which will be output
along with the pandoc output.
Docx reader: change tests to make use of media bag.
Shared: Make MediaBag available through Shared.
Pandoc.hs: change BSReader to output MediaBag as well as pandoc.
Make toplevel pandoc bin make use of mediabag.

Note that at the moment the mediabag is discarded. This will have to be
changed to make use of it.
Merge pull request #1468 from jkr/mediabag

Mediabag