Support markdown highlighting

Documentation blocks like `@doc` or `@moduledoc` [expected](https://hexdocs.pm/elixir/writing-documentation.html) to contain markdown templates.

I'm wondering, is it possible to integrate elixir-mode with mmm-mode to enable markdown editing for these blocks and describe setup in documentation (or make it enabled by default).

It would allow syntax highlighting, table editing and many more markdown features nicely integrated into working with `.ex` files.