Tune markdown separator and add mermaid

[jokester]

closes https://github.com/jokester/slides.ihate.work/issues/1

• it was not meaningful to steal from https://github.com/evilz/vscode-reveal , the plugin keeps its own copy of dependencies in git repo , and is not using reveal.js options directly

  • instead tuned markdown slide separators

• add https://github.com/zjffun/reveal.js-mermaid-plugin for mermaid