Support frontmatter-style title for link label autocomplete

[athyfr]

Those familiar with Pandoc (including myself) may prefer an alternative syntax for defining main file headers - the frontmatter:

---
title: Title of the Document
---

The lack of support is problematic for me, as the name of the document in the autocomplete ends up being the first level 2 heading.

Support for frontmatter may bring forward the possibility for a version of #216, as well.