added keys option to XAB
added keys option to XAB

Added two optional parameters to the XAB plugin so that they key that
subjects press to answer left/right is a parameter. The parameters have
default values of 'q' and 'p' to maintain compatibility.

vergenzt/jsPsych,Scienthusiast/jsPsych,msrresearch/jsPsych,adrianoesch/jsPsych,andyh616/jsPsych,jspsych/jsPsych,jspsych/jsPsych,andyh616/jsPsych,adrianoesch/jsPsych,erikasf/jsPsych,wolfgangwalther/jsPsych,shamrt/jsPsych,Scienthusiast/jsPsych,shamrt/jsPsych,jspsych/jsPsych,erikasf/jsPsych,jodeleeuw/jsPsych,vergenzt/jsPsych,jspsych/jsPsych,wolfgangwalther/jsPsych