Support class on fence as an alternative to code block detection

[jedelson-pagerduty]

While the way that this addon tries to auto-detect mermaid diagrams in code blocks is very nice and appreciated, would it be useful to also provide a way to bypass that detection using the mermaid class on the code block?

[johanneswuerbach]

Supporting an explicit class to enable rendering sounds good 👍, the auto-detecting will likely always lag a bit behind.