Add option for dynamic volume, independent of other features

[ewjmulder]

Everything already seems dynamically calculated, but it makes sense to have separate control over the volume of the audio resource. This can simulate things getting closer or further away or something turning up / down volume, talking softer / louder, etc.

Related to this is issue #25, that deals with dynamic volume based on distance. The same problems apply here, namely that in the current setup the normalizing process might be in the way. Or both this and #25 should be realized by some post-processing. That could work for this one, but for #25 we need the distance which might be lost by the time of post processing. Although recalculating is not that hard of course.