Merge pull request #1790 from mpickering/master

Text.Pandoc.Pretty: Improve performance of realLength
Text.Pandoc.Shared: Add hoist function

This is useful for lifting overly specific already existing parsing
functions so they can be used with more general monad stacks.
Text.Pandoc.Readers.Markdown: Generalise Type signatures
Text.Pandoc.Readers.Markdown: Add fenced div blocks

This is a speculative change which is meant to move discussion forward.
Updated README to explain fenced div blocks