[Feature request] Adds mmd (mermaid) file type support.

[jcvanhalle-ec]

Hi,

Support for rendering mermaid diagrams in markdown has been added #96 (pretty cool !).

Could we have diagram rendering when gist file extension is mmd ? That would remove the need to surround mermaid code with a markdown fence block when creating a new gist and copying code from mermaid live editor (and vice versa: it would remove the need to remove markdown fence block from opengist when pasting to mermaid).