Properly tokenize fence closing in GitHub style code blocks

[jonatanklosko]

Currently Markdown tokenizer marks both opening and closing ``` as string. However, if the opening one includes language as in ```elixir, then the closing one is incorrectly tokenized as variable.source.

[hediet]

Thank you for you PR!

Why is variable.source wrong though?

[jonatanklosko]

Currently the closing fence is tokenized differently if there's a langauge, for example:

Except this specific case it's tokenized as string.

[hediet]

Makes sense, thanks!