Match closing fence char and length in fenced code blocks

[martincizek]

is_codefence() now allows fence-pair matching to comply with https://spec.commonmark.org/0.29/#fenced-code-block

closing code fence of the same type as the code block began with (backticks or tildes), and with at least as many backticks or tildes as the opening code fence

So the following now works (as it does in GitHub, otherwise it would be difficult to type this :-))

````
This is some code containing a fence string
```
protected by extending the enclosing fence
````

~~~
This is some code containing a fence string
```
protected by using the other fence type
~~~

Fixes #208.

[mattr-]

Apologies for taking so long to review this and merge it in. Thank you for contributing this feature to Redcarpet! ❤️

[djuber]

@mattr- thanks for merging this! Is there a gem version release expected soon that incorporates this? Would you like me to start a PR for that?

[mattr-]

We'll handle the gem version bump and release from here. No ETA on when that might be though. If you can use master for the time being, I recommend doing that.