Add recommonmark AutoStructify transform and mermaid rST extension

This patch allows including the following CommonMark code block extensions in the documentation:

Math

```math
E = m c^2
```

E = m c^2

Admonitions

``` note::
   Its a note! in markdown!
```

Mermaid diagrams

``` mermaid::

   sequenceDiagram
      participant Alice
      participant Bob
      Alice->John: Hello John, how are you?
      loop Healthcheck
          John->John: Fight against hypochondria
      end
      Note right of John: Rational thoughts 
prevail...
      John-->Alice: Great!
      John->Bob: How about you?
      Bob-->John: Jolly good!
```

   sequenceDiagram
      participant Alice
      participant Bob
      Alice->John: Hello John, how are you?
      loop Healthcheck
          John->John: Fight against hypochondria
      end
      Note right of John: Rational thoughts <br/>prevail...
      John-->Alice: Great!
      John->Bob: How about you?
      Bob-->John: Jolly good!