Collect unparsed comments to rendered object for presenter notes

[yhatt]

To implement presenter notes, we are planning that HTML comments whose not to recognize as YAML object for directives will collect and return in the result object of Marpit.render().

<!-- theme: test-theme -->

# First page

<!-- This HTML comment is not YAML, so it would be collected. -->

---

## Second page

---

### Third page

<!-- It would be collected per pages. -->
<!-- These are expected to use as presenter note. -->

{
  html: '<div class="marpit"><section>...</section></div>',
  css: '.marpit > section { ... }',

  // Collected comments
  comments: [
    ['This HTML comment is not YAML, so it would be collected.'],
    [],
    [
      'It would collect per pages.',
      'These are expected to use as presenter note.',
    ],
  ],
}

At first we had planned to implement this feature in marp-core. But to decide a specification of comment in framework-level is better than implementation at inherited class.

Related to yhatt/marp#30.

[jasonwilliams]

Hey @yhatt do you know if speaker notes can be styled?
They come out as times new roman and small font and i wonder if its possible to add any styling to them?

[yhatt]

@jasonwilliams For now, the presenter note is the out of scope for theming provided by framework. Probably we need to update styling in Marp CLI's bespoke template: https://github.com/marp-team/marp-cli/blob/master/src/templates/bespoke/bespoke.scss#L243