Added writerMediaBag to WriterOptions.
pandoc: Thread media bag into WriterOptions.

This will make it available to docx and epub readers, etc.,
so we don't have to extract media to a directory when going
from docx -> epub.
Moved MediaBag back from Shared to Options, to avoid module cycle.
Shared:  Added fetchItem', which searches a media bag too.
PDF, Docx, EPUB, and ODT writers now automatically use MediaBag.

The MediaBag is thread through from the reader, with no need
to extract to files.