Support multiple blockquote levels

[coreagile]

I'm building an app that allows people to reply to messages. The serialization format is markdown. When someone replies to a message we automatically generate a blockquote like so:

> test
> 
> > test
> > > Welcome to the server.

Which should look like this in the editor:

test

test

Welcome to the server.

But instead it only quotes the first level.

The editor should render appropriately, and also allow people to manipulate the level of the blockquote.