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`.
  * Replace `MIME` defined in `Reader.EPUB` with `MimeType` from `Text.Pandoc.MIME`.