Stag should generate Kotlin code for Kotlin models

[anthonycr]

Summary

The only code generated by Stag is java code. This is all fine and good, but it prevents us from leveraging special features of the kotlin language such as internal visibility. We should be able to generate

Proposed Spec

• Split up stag-library-compiler into stag-compiler, stag-java-compiler, and stag-kotlin-compiler.
• stag-compiler will be an internal use only module and will house common logic. It will perform code generation via an interface or will provide data to an upper level which will perform the code generation instead
• The language specific modules will perform the code generation and will be what consumers of the library rely on.