Mediabag

These changes implement the MediaBag, as discussed today in email, in the Docx reader.

I've made the changes all through, in order to allow building and testing. Only the first two are related to the docx reader, so any subset can either be cherry-picked or resubmitted if the changes are being put through elsewhere.

Note that at the moment, the command-line option is not implemented. `pandoc.hs` just discards the MediaBag.