Rudimentary kotlin support.

Created by: dsyang

Right now this change doesn't to much more than enable kotlin_library() and kotlin_test() build rules. You will still need to include the kotlin runtime as a separate prebuilt_jar() build rule or else your kotlin targets will crash and burn.

There isn't a kotlin_binary() target yet. Use java_binary() instead and remember to add the suffix 'Kt' when specifying the main class (see https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html)

To use this you will need to:

• Have kotlinc in your path or specify it in your .buckconfig

[kotlin]
   compiler = ...

• Copy the kotlin-runtime.jar into your repo (intellij can do this for you when setting up kotlin) and create a prebuilt_jar() build rule for it. Include this target as a dependency under your kotlin_library() target.