Make defaults options editable

[jvolker]

As requested in OpenframeProject/Openframe#82 it seems like a good idea to make those default options editable.

I suggest a similar approach to what has been done in the slideshow extension – prioritising settings like this: per artwork > frame settings > coded defaults

https://github.com/jmwohl/Openframe-Slideshow/blob/master/extension.js

function getDuration(frameState, artwork) {
            var duration;
            if (artwork.settings && artwork.settings[pjson.name] && artwork.settings[pjson.name].duration) {
                duration = minToMillis(artwork.settings[pjson.name].duration);
            } else if (frameState.settings && frameState.settings[pjson.name] && frameState.settings[pjson.name].duration) {
                duration = minToMillis(frameState.settings[pjson.name].duration);
            } else {
                duration = minToMillis(DEFAULT_DURATION);
            }
            return duration;
        }