Allow Markdown H1 to set titles

[jacobb]

Currently, titles are based on frontmatter. If there is no title frontmatter, we default to the filename.

H1 headers should be included in this. I think the order of preference is likely:

1. Frontmatter title, if present
2. First H1 Header
3. (maybe) Top-most header, regardless of header level
4. the filename, minus extension (.md)