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.
Updated tests.
More test updates.
Added network-uri flag.

This addresses the split of network and network-uri.
Merge pull request #1553 from mpickering/master

Txt2Tags reader: Header is now parsed only if standalone flag is set
Fixed --self-contained with Windows paths.

Previously C:\foo.js was being wrongly interpreted as a URI.
Closes #1558.
Docx writer:  Bibliography entries get Bibliography style.

Closes #1559.
Fix build dependencies for the trypandoc flag.

Build dependencies of the trypandoc executable are required, regardless of the
trypandoc flag was set to either True or False.  Correct package description
to make them truly optional.
Capitalized Build-Depends for consistency.
Hlint changes to Docx writer.
Docx writer: Implement track changes.

These have default authors and dates of "unknown" and timestamp-zero,
respectively.
Docx writer: Default to user login and time of change if not given.
Docx writer: Accomodate GHC 7.4 (no lookupEnv)
Fixed exampleLine parser to accept example lines which have indentation at the start of the line.
Merge pull request #1565 from Chobbes/master

Org exampleLine parser accepts indented example lines!
Merge pull request #1564 from jkr/trackChangesWriter

Docx writer: write track changes.
DokuWiki Writer: Qualified all imports
DokuWiki Writer: Hlint cleanup