Add Mermaid diagram support

[jforman]

Is your feature request related to a problem? Please describe.
I love the anatole hugo template, and the ability to have a wide content area. I would like to include Mermaid diagrams (https://mermaid-js.github.io/mermaid/#/) as part of my posts which often contain technical information where a diagram is incredibly helpful to have.

Describe the solution you'd like
Ability to enable mermaid support in the config (so that those who dont want to load the JS dont need to), and then include mermaid diagrams in their posts.

Describe alternatives you've considered
None

Additional context
I've tried a pull request to add the support at #359, but I'm not sure I'm understanding the templating system quite right, with the hugo documentation (https://gohugo.io/content-management/diagrams/#mermaid-diagrams). The netlify tests so far are failing with my PRs.

Thanks,

[jforman]

I believe I have mermaid support correctly added with my updates to #359. Thanks!