Adds `mermaid.js` and `highlight.js` for code formatting along with `Fire` managed args to mkposter

[stanbiryukov]

Adds:

• mermaid.js diagram support for rendering mermaid blocks
• code rendering with highlight.js, default github theme
• pass arguments to mkposter using Fire
  • adds background_color str argument with default #FFFFFF
  • adds code_style str arg for highlight.js themes with default 'github'
  • adds port int arg to specify http.server port, with 8000 as default.

Example:

python -m mkposters "research_app/poster" --code_style "github" --background_color "#F6F6EF" --port 8000