Mermaid default startOnLoad does not work on code blocks with syntax highlighting

[chrisirhc]

A code snippet contains syntax highlighting, it currently will fail to render with the default integration.

Example: https://codepen.io/chrisirhc/pen/bNGRvZj

I propose that the default integration make use of textContent of the target element instead of the innerHTML which contains html tags.

Why

Currently, bloggers need to choose between syntax highlighting mermaid diagrams codes or rendering the diagrams. They can't easily have both unless they do some additional customisations.