Toolbar button for inserting a horizontal rule

**- Do you want to request a *feature* or report a *bug*?**

*feature*

**- What is the current behavior?**

There is no way to add a horizontal rule (`---`) using the markdown widget in rich text mode, except by implementing it as a custom editor widget. I would like to have that so I can use it as a structural divider for some content that I'm managing via netlify-cms.

The editor does have good support for the horizontal rule as soon as it's there, though. It's rendered as a horizontal line that goes all the way across the editor, and it compiles to an `

**- What is the expected behavior?**

That there is a toolbar button with an "—" icon or similar that inserts a `---` on an empty line into the markdown.

It seems like slate.js does not have this built in, but we could probably upstream that.