Add 'code' as alternative to 'code-block'.

[terrencegf]

I have seen several RST markdown files where 'code::' is used instead of 'code-block::'. This causes the library to throw an error about 'unknown directive'. So simply copy Directives/CodeBlock.php to Directives/Code.php and add Directives\Code to Kernel.php to allow ignorant users to use 'code:'', even if it is incorrect.