Add blockquote fence syntax to Markdown

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/16564

Before Markdown rendering happens, this will transform this:

Let me quote this here email:

>>>
Dear friend,

How are you?

Greetings,

Me
>>>

Into this, saving me from having to prefix all of those lines with > manually when I copy some multiline text from another medium:

Let me quote this here email:

> Dear friend,
> 
> How are you?
> 
> Greetings,
> 
> Me

Markdown rendering will then make it all pretty:

---

To do:

• Ensure this doesn't transform code within code fences (```)
• Write documentation
• Write tests