title: sbt: run task on subproject
body: <p>I have the following project structure:</p>
  
  <pre><code>lazy val root = project.aggregate(rest,backend)
  lazy val rest = project
  lazy val backend = project
  </code></pre>
  
  <p>When I execute the "run" task from the parent, I want a specific class from the "backend" project to have its <code>main</code> method executed.  How would I accomplish this?</p>