Add an option to wrap indented code parts into language specific code blocks

[RobertDober]

What

Now and without the option the markdown from docstrings is copied verbosly e.g.

              iex(42)> some_doctest
             42

but when issuing e.g. `<%= xtra.moduledoc "MyModule", code_block_language: "elixir" the docstrings will be processed to yiedl

             ```elixir
              iex(42)> some_doctest
              42
              ```

Rationale:

This will generate the same syntax highlighted output in README.md on Github that the docstrings yield on hexdoc.pm