(arg → string, classId → string, callback → function)
- register inView handler
function (arg, classId, callback) {
  var view = inView('.' + classId)
  arg === 'on' ? view.on('enter', callback.enter).on('exit', callback.exit) :
  arg === 'once' ? view.once('enter', callback.enter).once('exit', callback.exit) :
  console.warn('[in-view] event handler not found')
}