Output HTML per slide page as array by passing htmlAsArray env

[yhatt]

By passing htmlAsArray: true env when calling render(), Marpit would become to output HTML per slide page as array.

const markdown = `
# Page 1

---

# Page 2
`

const { html } = new Marpit().render(markdown, { htmlAsArray: true })

/*
[ '<section id="1">\n<h1>Page 1</h1>\n</section>\n',
  '<section id="2">\n<h1>Page 2</h1>\n</section>\n' ]
*/

Marpit initializer's container option is still working, so you have to render these HTMLs within <div class="marpit"> or customized element.

Resolve #112.