Bug/Request: quad-backtick code blocks in markdown lexer

[Ravlen]

The markdown lexer should allow code blocks that have quad backticks, which is used when you need to encapsulate a triple backtick codeblock. For example, you can see it works fine in GitHub:

This is how you create a codeblock in markdown:

```yaml
---
one: Mark McGwire
two: Sammy Sosa
three: Ken Griffey
```

This is more text.

As you can see, it doesn't work with Rouge:

[pyrmont]

I think this should probably be marked as a bug. The CommonMark† spec provides that fenced code blocks can contain more than 3 tildes.

† I realise that CommonMark and Markdown are technically different but since CommonMark is the one with a proper spec, it seems logical to use it when trying to write a lexer.

[Ravlen]

Yeah, after I clicked submit I realized I should have started with the bug template, thanks for updating it for me!