Add a kotlin compilation step

Build.scala

val main = PlayProject(appName, appVersion, appDependencies, mainLang = SCALA).settings(
  
  compile in Compile <<= (compile in Compile, sourceDirectory in Compile, classDirectory in Compile, dependencyClasspath in Compile) map { (analysis, src, classes, classpath) =>
    
    val allKotlinSources: Seq[java.io.File] = (src ** ".kotlin").get
    
    // call the kotlin compiler using `classpath` and output classes to `classes`
    
    analysis
  }
  
)