watch d_lang source use sbt

build.sbt

{
val script = "hello.d"
seq(
  TaskKey[Unit]("dmd") <<= (compile in Compile).map{ _ =>
    val ret = ("dmd -run " + script) !;
    if(ret != 0)sys.error("compile fail")
  }
  ,
  watchSources += file(script)
)
}