Expose selected size to the parent of each slide

[yhatt]

This idea came from #134.

By exposing selected size as data attribute of <section> slide container, theme author can change style by size.

---
theme: xxx
size: A4
---

# Hello!

<section data-size="A4">
  <h1>Hello!</h1>
</section>

/*!
 * @theme xxx
 * @size 4:3 960px 720px
 * @size A4 210mm 297mm
 */

section {
  /* Default style (16:9) */
}

section[data-size="4:3"] {
  /* Style for 4:3 slide */
}

section[data-size="A4"] {
  /* Style for A4 slide */
}

It might be better to work at Marpit framework, to expose the value of global directives as like as local directives.