Populate new journal entry with YAML frontmatter

[bsima]

Most markdown parses include YAML frontmatter, which jour uses for tags:

---
tags: work, tea
---

Today I worked on code stuff while drinking tea...

jour ls then prints this as

2017.05.02 : work, tea

which is nice. It would be great though if jour automatically put the YAML frontmatter in the $EDITOR buffer when I started writing a new journal entry.