Add support for multiple disposers

[pathikrit]

e.g.

def run(inputDir: File, outputDir: File, partitions: Int = 10) = {
    implicit def disposeMultiple[A](c: Disposable[A]): Disposable[IndexedSeq[A]] = Disposable(_.foreach(c.dispose))
    val lineCount = new AtomicInteger
    for {
      writers <- IndexedSeq.tabulate(partitions)(i => (outputDir / s"part-$i.csv.gz").newGzipOutputStream().printWriter()).autoDisposed
      file <- inputDir.list(filter = _.extension == Some(".csv")).toSeq.par
      line <- file.lineIterator.drop(1)
    } writers(lineCount.incrementAndGet() % partitions).println(line)
  }