title: Pass argument to sbt task
body: <p>I have the following sbt task that I'd like to pass an argument to:</p>
  
  <pre><code>import sbt.complete._
  import complete.DefaultParsers._
  
  val stageEnv: Parser[String] = " staging" 
  val prodEnv : Parser[String] = " production"
  val combinedParser: Parser[String] = stageEnv | prodEnv
  
  slick := slickCodeGenTask.value
  
  lazy val slick = inputKey[InputTask[Seq[File]]]("gen-tables")
  
  lazy val slickCodeGenTask = Def.inputTask {
    ...
    s.log.info(combinedParser.parsed.trim)
    Seq(file(fname))
  }
  </code></pre>
  
  <p>However, when I run it and pass an arg like 'sbt slick prodEnv' I get the following error:</p>
  
  <pre><code>[error] Not a valid command: prodEnv
  [error] Not a valid project ID: prodEnv
  [error] Expected ':' (if selecting a configuration)
  [error] Not a valid key: prodEnv (similar: products)
  [error] prodEnv
  [error]  
  </code></pre>