LaTeX writer: use \(..\) instead of $..$ for inline math.

Closes #1464.
Docx writer:  Print subtitle from metadata if present.

Use Subtitle style.

See #1451.
Mediawiki writer:  don't escape inside `<source>`.

Closes #1445.  Escapes can still be used with `<code>`
and `<pre>`.
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
Added --extract-media option.

This has been documented to affect the epub and docx readers, so
we should either add the epub reader before the next release or
change the documentation.
Moved withTempDir from PDF to Shared, export from Shared.

API change.
Moved MediaBag from Shared to Options.

This will allow us to put a MediaBag in WriterOptions.
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.
RTF writer: refactored image embedding, using fetchItem'.
RTF writer:  Improved image embedding.

Use calculated sizes.
Allow --self-contained to get content from MediaBag.

Added a parameter to makeSelfContained (API change).