Add block quote support

[morinted]

Jira and Markdown have block quote support.

E.g.

bq. This is a block quote in Jira

> This is a block quote in Markdown

There's also multi-paragraph block quoting:

{blockquote}
There is so much text to talk about.

You can't get over it.
{blockquote}

> There is so much text to talk about.
>
> You can't get over it.

[FokkeZB]

Good one. Probably not too hard to implement with regexes, although I'd prefer to solve this via #12. Unfortunately I don't have time for that atm. I'm open for a PR of course!