mermaid: classDiagram breaks when using <<interface>>

[koenyskout]

When I create a mermaid diagram as follows (which works fine in mermaid's live editor), mermaid shows a "Syntax error in text" error on my hugo site.

```mermaid
classDiagram

class Foo {
  +a: int
}
<<interface>> Foo

The `<<interface>>` line is the culprit. I assume this is due to some HTML sanitization issue, where `<interface>` might be misinterpreted as a HTML tag.

[McShelby]

Thanks for submitting. I fixed it pushed it. Check it out.

[koenyskout]

Works perfectly, thanks!