Collect HTML comment per pages to be usable as presenter note

[yhatt]

This PR will collect HTML comment per pages (except YAML for directives) and return to comments member in Marpit#render after rendered. It will expect to be used as presenter notes in integrated apps.

By this change, comments member is added to the result of Marpit.render(). It's a double-dimensional string array.

const md = `
<!-- theme: default -->

# Hello, world!

<!-- Comments (for presenter note) -->
<!--
To keep a compatibillity with general markdown document, we're using HTML
comments to be realize the presenter note of slide deck.
-->

---
<!-- backgroundColor: black -->

## Marpit framework

<!-- It would collect HTML comments except YAML for directives. -->
`
const marpit = new Marpit()
const { comments } = marpit.render(md)
console.log(comments)

[ [ 'Comments (for presenter note)',
    'To keep a compatibillity with general markdown document, we\'re using HTML\ncomments to be realize the presenter note of slide deck.' ],
  [ 'It would collect HTML comments except YAML for directives.' ] ]

Currently Marpit won't handle the parsed comments and treat as plain text(s). The integrated app can use comments as presenter note, and you can be post-processing by another markdown parser if you want any styling.

Closes #82.

ToDo

• Update TypeScript definition
• Update JSDoc
• Update docs
• Update CHANGELOG.md