Hide module Text.Pandoc.Highlighting.

This is really an auxiliary module.
Version bump to 1.11 due to API changes.
Revert "Hide module Text.Pandoc.Highlighting."

This reverts commit 01753ead71f6a08929f1535fcb497ff6d6015289.

We need it after all in pandoc.hs.
Added `--default-image-extension` and `readerDefaultImageExtension`.

Note: Currently this only affects the markdown reader.