Support header on first line of Markdown as title

[rgaiacs]

When

---
title: Title goes here
---

Some text.

is converted to Word, we have

But when

# Title goes here

Some text.

is convert to Word, we have

As reported in stencila/encoda#92, users might expect that the output for both Markdown documents will be the same.

[nokome]

Just for reference, this is related to this discussion and the interpretation of heading depths and the generation of HTML5 document outlines by the html codec. All needs further thought but a simple hueristic would be:

• if the Markdown has a title in the YAML metadata then use that
• if the first item in the content is a Heading with depth:1 then use that as the title and remove it from the content (should all other headings then be promoted by one i.e. depth: Math.max(1, depth - 1) ?)