support msgpack format

[acidtib]

If using :msgpack as a format this error is raised Exception: Missing named tuple key: :msgpack (KeyError)

this pr adds msgpack as a format

Kave.configure do |c|
  c.format = :msgpack
end

[jwoertink]

Hey! thanks for submitting this. Definitely one thing I wanted to add. One thing though is that if you're using msgpack, you'd still have to pass in a json accept header, so https://github.com/jwoertink/kave/blob/master/src/kave.cr#L7 this would need to be updated as well. If you figure that out, can you also add a spec to check if it's json or msgpack to make sure it doesn't break the json side?