add block_index to default data
add block_index to default data

records the index of the current block
Scienthusiast/jsPsych,erikasf/jsPsych,wolfgangwalther/jsPsych,shamrt/jsPsych,erikasf/jsPsych,vergenzt/jsPsych,Scienthusiast/jsPsych,adrianoesch/jsPsych,jspsych/jsPsych,wolfgangwalther/jsPsych,msrresearch/jsPsych,vergenzt/jsPsych,jspsych/jsPsych,shamrt/jsPsych,jspsych/jsPsych,jspsych/jsPsych,jodeleeuw/jsPsych,adrianoesch/jsPsych,andyh616/jsPsych,andyh616/jsPsych