Does not work with code nested in blockquote

[karelbilek]

Minimal example:

Input:

> ```
> Foo
> ```


```
foo
```

Output:

> ```
> > Foo
> > ```
>
>
> ```
>
> foo \`\`\`

[karelbilek]

It seems that it works when both code blocks are at the same level - that is, if both are in blockquote, or both outside of it.

But once one is inside blockquote and one outside, either blackfriday or markdownfmt loses its marbles

[karelbilek]

#56