Support latest es module mermaid, add support for ELK diagrams

[timkpaine]

• support es module mermaid with switch for allowing older non-es module
• rework d3 zoom for new mermaid
• add optional (but defaulted) ELK support
• switch from unpkg to jsdelivr
• cleaner options for local mermaid, mermaid ELK, d3 (fixes Allow use of local d3 distribution #142)
• pass-through mermaid frontmatter (fixes Support YAML frontmatter configuration #147)

ES Modules

Mermaid moved to es modules on version 10.3. As a first pass, I allowed for either >10.3 with code to use modules, or less than 10.3 and keep the existing code. This ended up pretty messy, so this PR officially removes support for mermaid<10.3.0. This is a breaking change.

Explicit local files

I added explicit options for mermaid_use_local, mermaid_elk_use_local, and d3_use_local, to disambiguate the usage of mermaid_version.

New Zoom

This is a little more complicated because I noticed a race condition between mermaid's auto initialization and the d3 processing step on window load. So now they're guaranteed sequential by forcing mermaid to run first, then doing a followup run to add zoom to either all mermaids on the page, or just those that have been annotated with :zoom:

Frontmatter

Mermaid frontmatter is working, albeit with a compromise of exploiting yaml/json compatibility. Thankfully, this works fine across sphinx/github:

```mermaid
---
title: Hello Title
config: {"theme": "base", "themeVariables": {"primaryColor": "#00ff00"}}
---
flowchart
	Hello --> World
```

GitHub

---
title: Hello Title
config: {"theme": "base", "themeVariables": {"primaryColor": "#00ff00"}}
---
flowchart
	Hello --> World

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Sphinx

Docs

I validated that html/pdf docs worked locally including zoom. I added some information:

• new configuration options
• how to make github/sphinx treat mermaid directive the same (e.g. s/{mermaid}/mermaid)

[mgaitan]

@timkpaine amazing . could you update the README and maybe add an example to the doc?