Support programmatic configuration of Kotlin plugin

[cbeams]

When this work is complete, it will eliminate the current need for a static gradle.ktscfg.xml (#19) file to be generated (by generateKtsConfig) at the root of each project.

The general idea is to expose a configuration API on the Kotlin plugin side and to then integrate with it via the Gradle Tooling API.

[cbeams]

@ligee, could you provide an update on this? We're considering cutting M2 on Monday the 20th (a couple days earlier than planned). Can this issue still make it?