added options to si-kr plugin
added options to si-kr plugin

can now opt to leave the stimulus on the screen for a fixed length of
time, still recording the responses when they occur.

also allows a timelimit to be set on the responses, so that if no
responses is recorded after some period of time, the trial completes and
records a -1 as the response.

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