Allow functions for individual elements in data parameter

[jodeleeuw]

Allow things like:

var trial = {
  // ...
  data: {
     round: function() { return round_index; },
     type: 'foo'
  }
}

[wolfgangwalther]

And while at it maybe a helper function that could replace code like this:

var trial = {
  data: {
    a: jsPsych.timelineVariable('a'),
    b: jsPsych.timelineVariable('b'),
    c: jsPsych.timelineVariable('c'),
    d: 'static'
  }
}

with this:

var trial = {
  data: jsPsych.HelpMeGetAllTimelineVariables({
    d: 'static'
  })
}

This helper would then merge the object given as argument with all available timelineVariables.