mermaid: refactor config to use mermaid_config.js include, only require mermaid.version in _config.yml (#909)

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!

_layouts/default.html

@@ -215,7 +215,11 @@ layout: table_wrappers
     {% endif %}
   </div>
 </body>
-{% if site.mermaid_enabled != false %}
-  {%- include mermaid_init.html -%}
+{% if site.mermaid %}
+  <script>
+    var config = {% include mermaid_config.js %};
+    mermaid.initialize(config);
+    window.mermaid.init(undefined, document.querySelectorAll('.language-mermaid'));
+  </script>
 {% endif %}
 </html>