Accept pandoc-style YAML frontmatter

Pandoc (and some other tools) have decided to end their YAML frontmatter
with `...` instead of `---`.  In the name of flexibility, this patch
allows either to mark the end of YAML frontmatter.

Example:

```
---
title: No place like home
...
```
Merge pull request #1350 from docwhat/pandoc-frontmatter

Accept pandoc-style YAML frontmatter