code: function tasks (taskName) {
    var taskList = _.keys(gulp.tasks)
  
    if (taskName == null) {
      return taskList
    }
  
    return _.filter(taskList, function (task) {
      return _.startsWith(task, taskName)
    })
  }
docs: Returns every gulp tasks with a name that starts by taskName
  if provided, otherwise returns all existing tasks.
  
  @param {String} [taskName]
  @returns {Array}