Add jspsychdemo about jatosr

[jirilukavsky]

In a dedicated branch b51e98c, there is a implementation of raw_json and raw_jspsych. The function raw_jspsych works as described above, with additional option to work on process_results list (not only process_results$data.

Not sure about the names - the current names raw_json and raw_jspsych are simple. Potentially, convert_from_json or convert_from_jspsych might be also good. Note that "process" verb is already used in jatosR::process_results and the words "json" and "jspsych" are not output formats, but rather input format (json) or tool producing the data (jspsych), which should be considered when adding some prepositions ("as_json", "to_json", etc.).

[jirilukavsky]

For now, we keep the simple names raw_json or raw_jspsych.

[jirilukavsky]

Since version 0.3.0 895da1b