survey-multi-choice does not record option indices

[wilbertvanham]

survey-multi-choice does currently not record the index of the option selected. This makes a trial like this, with multiple identical strings in the options array, useless.

type: jsPsychSurveyMultiChoice,
questions: [
  {
    prompt: "How much", 
    options: ["Little","","","","Much"],
  }
]

In my opinion not relying on the string in data analysis is better since it allows for instance for internationalization. It would also solve the issue that an unanswered question is indistinguishable from choosing an empty string option now.

Adding a response_index array in the data would be a downward compatible way to solve this issue.

"response": {
	"question1": "Little",
	"question2": "",
	"question3": "",
	"question4": ""
},
"response_index": [
	0,
	1,
	2,
	3
],
"question_order": [
	0,
	1,
	2,
	3
],