MIME cleanup.

  * 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`.
Update Reader.EPUB to use `MimeType`.
Remove an unnecessary import.
Merge branch 'mime' of https://github.com/Aelve/John into Aelve-mime

Conflicts:
	src/Text/Pandoc/Writers/Docx.hs
Updated changelog.