Allow use literal blocks (::)

[kataev]

For some people using ..code-block not practical. For example https://github.com/zzzeek/sqlalchemy/pull/469#issuecomment-443554287

Need add functionality for parsing literal blocks directive: http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks

[afeld]

On the other hand, literals may be used for things other than Python code, so not sure it should be enabled by default.