Add injectable scripts and events

[PlytonRexus]

• We can probably use events to signify when something, say, a variable, changes or a choice is selected or a scene starts. At such times, an event can be fired and used to create changes. Say, the background can be changed or a piece of music can be started, or text can flicker and so on ...
• On similar lines, we can - later - introduce all these features via if-script syntax. But for now, at least, maybe we can add injectable JS that can carry listeners for the events that we fire. Besides, listeners, these scripts can contain the customisations that the authors may desire. One thought on doing so is to add a function injectible(callback(state)). Authors must pass the callback that will receive the current game state and be executed.