Unselect feature

[himanshi-2602]

At present the Sunbird QuMl player doesn't support the unselect feature i.e for MCQ-type questions if the player chooses a particular option and after some time, he/she realizes that the selected option is not correct and wants to skip that question but the QuMl player will not allow him/her to do so. So, once you select an option, you can change your selected option but can't skip that question. This unselect feature will be helpful in those assessments which impose negative marking on the wrong answer.