R1:80484b76a5ff

Remarkup Code-block: parse language specifier in markdown

Summary:
We add support to code blocks with the language expressed as GitLab/GitHub/StackOverflow/...
"flavored markdown".

So we support this syntax: (to avoid confusion see it online on the Diff)

lang=text
```php
$asd = 1;
```

Before this change, this was the only supposed syntax in Remarkup, with an explicit "lang=":

lang=text
```lang=php
$asd = 1;
```

This change introduces a minor risk to eat legitimate…