Support horizontal rules

[brianlovin]

It would be great to have the same markdown treatment for horizontal rules where three hyphens, asterisks, or underscores will introduce an

---

tag.

Ref: https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet#hr

[michaelgmcd]

Looked briefly into this and it appears that the draftail editor supports horizontal rules:

Some relevant pieces of code:

• https://github.com/springload/draftail/blob/8c1625af2368f4dd8c2a582f12da063047fa2ec4/lib/api/DraftUtils.js#L114-L124 .
• https://github.com/springload/draftail/blob/8c1625af2368f4dd8c2a582f12da063047fa2ec4/lib/components/DraftailEditor.js#L417-L424