Add header and footer directives

[yhatt]

This PR adds header and footer local directives.

---
header: "Header content"
footer: "Footer content"
---

# Slide contents

The content specified in a header directive is added to first of each slide with wrapping by <header> element. The content of a footer directive also adds to last of each slide with wrapping by <footer>.

<section>
  <header>Header content</header>
  <h1>Slide contents</h1>
  <footer>Footer content</footer>
</section>

Marpit just inserts corresponding elements. When you want to place these contents in the marginals of slide, the using theme must support fixed position. If not, you would simply see these contents as the part of slide content.

Unlike the footer directive in pre-released Marp, Marpit's directives support markdown syntax for inline text. You can style text as **bold**, _italic_, and so on.

ToDo

• Add test case about inline markdown syntax
  　
• Update README.md
  • header and footer directives
  • How to style these contents on theme