jvm_target doesn't support modern java

[kotlinc supports up to java 12](https://kotlinlang.org/docs/reference/compiler-reference.html#-jvm-target-version) as a `jvm_target`, but `rules_kotlin` is [limited to java 1.8](https://github.com/bazelbuild/rules_kotlin/blob/master/kotlin/internal/toolchains.bzl#L138-L145). It would be very helpful if the default values included the options that can be passed to the compiler.