Syntax error occur when multiple mermaid diagrams are included

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Summary

If multiple mermaid diagrams are included in markdown file, a syntax error will occur.

Reproduce

# test

```mermaid
stateDiagram
  s1-->s2
```

```mermaid
stateDiagram
  s1-->s2
```

[erik-nilcoast]

I see this as well. I'm going to see if I can fix this locally. I also just added an option to specify the mermaid version.

[erik-nilcoast]

Here's a PR for setting the mermaid version: https://github.com/yusukebe/gh-markdown-preview/pull/63 and removes the copy paste buttons which were interfering with mermaid.