Add onLoad, preRun, postRun

  onLoad :: Node -> IO a
    parameter to bindSplice

  preRun :: Monad m => Template -> m a
  postRun :: Monad m => Template -> m a

  The difference between pre-and post-Run and the splice is that the
splice only gets spliced node as input.  preRun and postRun get the
whole document.