This PR has a bit of scope creep! This PR now:
- changes the mermaid opt-in logic to only check for the existence of a `mermaid` key instead of `mermaid != false`: this resolves the follow-up in #857
- changes the behaviour of mermaid configuration
- instead of using `mermaid_init.html` with default settings, makes the include `mermaid_config.js`
- the include is loaded directly into the contents of `mermaid_initialize`
- by default, it is an empty object (i.e. `{}`), triggering the defaults
- updates docs
- adds an example to the markdown kitchen sink
It does significantly change the interface provided in #857, and I apologize for the confusion. However, given the discussion in this PR, I think it's the best move forward!
