Enhancement: Demo notes shown in the presenter view

[estruyf]

Add the ability to make demo notes and show these in the presenter view.

Thoughts

Use Markdown in JSON or a separate markdown file linked to the demo ID.

Example of JSON

{
  "note": "# The notes\n..."
}

Example of the markdown file

{
  "notes": {
      "path": "demo1.md",
      "showOnTrigger": true
    }
}

The markdown file should be called: demo-1.md and put in a specific .demo/notes folder.