MediaBag: Improved normalisation when writing files
Merge pull request #1487 from mpickering/master

MediaBag: Improved normalisation when writing files
MediaWiki reader doesn't recognize german "Bild"
Merge pull request #1490 from ulikoehler/master

MediaWiki reader doesn't recognize german "Bild"
Docx reader: Use TeXMath to write math

The new version of TeXMath can translate from its type system into
LaTeX. So instead of writing the LaTeX ourself, we write to the TeXMath
`Exp` type, and let TeXMath do the rest.
Add a note on how `mapD` works.
Remove now-unnecessary TexChar

TeXMath does the work now.
Docx reader: remove now-unnecessary state variable.

This also introduces a `defaultDState` value.
Merge pull request #1491 from jkr/texmath-equations

Docx Reader: Use TeXMath for writing equations.
Get rid of unused docx variable.

Since changing the Docx type, this is no longer necessary. Thanks to
Matthew Pickering for picking up on this.
Merge pull request #1492 from jkr/nodocxenv

Get rid of unused docx variable.
pandoc:  Don't strip path off of sourceURL.

We need this information for relative URLs!

This should resolve the continuing problem noted in #750.
EPUB Reader: Improved how images are extracted
EPUB tests: Updated test file