(fix) YAML front matter must be the top of the file

[nobiot]

At the moment, YAML front matter can be placed anywhere in the buffer, and md-roam will pick up the title, etc. from it.

This is not good.

Pandoc document specifies the front matter is at the top of a file:

A YAML metadata block is a valid YAML object, delimited by a line of three hyphens (---) at the top and a line of three hyphens (---) or three dots (...) at the bottom.