Record the date as part of the frontmatter on newly created docs

[cjsaylor]

The open command (if the entry doesn't exist) should initialize the frontmatter with a date entry.

Ex:

---
date: 2018-07-26
---

the tag command should keep the date value unchanged and just append the tag entries:

---
date: 2018-07-26
tags:
- sometag

[cjsaylor]

Done.