Add support for case-in statements

[jdelStrother]

WDYT to adding case-in support?

- case type_and_details
- in [:user_account, user]
  Profile #{user.name}
- in [:post, post]
  Post #{post.title}

Let me know if you think it needs more tests. I looked at adding some to engine_tests.rb, but they seem more like regression tests, wasn't sure of the best place to add it.

[k0kubun]

Nice, thanks!