* Create a type synonym for MIME type (instead of `String`).
* Add `getMimeTypeDef` function.
* Avoid recreating MIME type `Map`s every time.
* Move “Formula-...” case handling into `getMimeType`.
src and poster will both be incorporated into content.opf
and the epub container.
This partially address #1170.
Still need to do something similar for <audio>.
* `Text.Pandoc.PDF` exports `makePDF` instead of `tex2pdf`.
(API change.)
* `makePDF` walks the pandoc AST and checks for the existence of
images in the local directory. If they are not found, it attempts
to find them, either in the directory containing the first source
file, or at an absolute URL, or at a URL relative to the base URL
of the first command line argument.
* Closes#917.
