Allow customization the content of pagination

[yhatt]

Marpit could not modify the content of pagination via content declaration in section::after. This PR makes it possible.

Theme CSS and tweaked style must contain attr(data-marpit-pagination) in content declaration of section::after because user expects to show the page number by paginate: true directive. Marpit will still ignore the whole of content declaration if the reference to that attribute is not contained.

Also we added data-marpit-pagination-total attribute, indicates the total page number of rendered slides , to section element. Of course, it can use in the content.

---
paginate: true
style: |
  /* Add "Page" prefix and total page number */
  section::after {
    content: 'Page ' attr(data-marpit-pagination) ' / ' attr(data-marpit-pagination-total);
  }
---

![bg 70%](marp.png)

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# Second page

This idea is came from yhatt/marp#263.