Mermaid diagram causing syntax error

[stigi]

Mermaid support was added in #96 and it's working in the global feed.

It doesn't work on the individual gist page though. Example: https://demo.opengist.io/stigi/53ccad130f6f4cc3b8d687af5209b2e3

Here's what the same gist looks like in the feed:

Content of the gist:

```mermaid
  graph TD;
      A-->B;
      A-->C;
      B-->D;
      C-->D;
```