Publish the cell and display layout API as a separate artifact.

[altavir]

I mean things like TypedMIMEContainer etc. It is required to be able to make plugins. I also think that there should be an abstraction for the current JSON module specification. So things like init code could be loaded directly from the jar.

[ileasile]

This thing was implemented, see https://github.com/Kotlin/kotlin-jupyter/blob/master/docs/libraries.md for documentation