Investigate/create Gradle plugin for configuration rather than Regex and compiler arguments

[alex2069]

Currently we try to guess the current build/variant etc using paths and Regex (CompileContext.kt), and allow compiler arguments if something goes wrong (Compiler.kt). This process is quite messy and fragile.

• Ideally we could use a Gradle plugin that extracts the appropriate info from the Android Gradle plugin (as the Kotlin Gradle plugin does)
• We will eventually want a Gradle plugin if we create Kotlin compiler plugins anyway (Investigate Kotlin Compiler plugin for generating/handling default value arguments (and injection?) #17)
• Possibly still keep the current resolution methods as a fallback?