Audio button response plugin missing "stimulus" column in data generated

[vzhang03]

Going through the code and documentation for the plugin-audio-button response, it was missing a "stimulus" column. Inconsistent with the other similar plugins (image-keyboard-response, html-button-response), this missing info seems easy to generate by using a combination of parameters such as stimulus (path to audio file) and button_html (HTML for generating buttons). Would help classify data output from the plugin by giving the user an idea of what response is responding to.