Support of mermaid blocks in .md files

[Valentin Shpak]

I've noticed an issue when trying to convert a .md file with mermaid code.
Mermaid is widely used in Markdown file and works well. Also mermaid support already added to Sphinx using the sphinxcontrib-mermaid plugin. Mermaid markup in a .rst file is look as below:

.. 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 <br/>prevail...
      John-->Alice: Great!
      John->Bob: How about you?
      Bob-->John: Jolly good!

But seems during composing the documentation from a .md file Sphinx not able to recognize mermaid markup because M2R converted 1st line of mermaid block not into .. mermaid:: string but into .. code-block:: mermaid string.
Could you please add conversion of mermaid block not as code-block but as separate and specific code?