Added Preformatted Block

Signed-off-by: Adam Engebretson <adam@enge.me>
Acked-by: Étienne Vallette d'Osia <evo@zengularity.com>
PreformattedBlock now support formatting

Formatting is not stored using markup, so there is no need to avoid it
in these blocks.
StructuredText blocks now escape their content
Merge branch 'adamgoose-master'

* adamgoose-master:
  StructuredText blocks now escape their content
  PreformattedBlock now support formatting
  Added Preformatted Block