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!

docs/index-test.md

@@ -71,7 +71,7 @@ end
 
 [This is a very long link which wraps and therefore doesn't overflow
 even when it comes at the beginning](.) of the line.
-  
+
 - [This is a very long link which wraps and therefore doesn't overflow the line
   when used first in an item ](.) in a list.
 
@@ -298,6 +298,19 @@ class conditions(object):
 Long, single-line code blocks should not wrap. They should horizontally scroll if they are too long. This line should be long enough to demonstrate this.
 ```
 
+### Mermaid Diagrams
+
+The following code is displayed as a diagram only when a `mermaid` key supplied in `_config.yml`.
+
+```mermaid
+graph TD;
+    A-->B;
+    A-->C;
+    B-->D;
+    C-->D;
+```
+
+
 ```
 The final element.
 ```