Add fx loop

adds Wad.prototype.setUpFxLoopOnPlay(that, arg, context)
User overrides this method in their own script, and either pushes on their own nodes or does their thing.

For the Tuna connection .input thing, I just did some fancy binding. I'd like to find a more practical example for this though, like maybe adding a raw web audio node of some kinda. Added an example to the Readme for using Tuna with Wad. Going to do a frequency analysis/fft example as well.

Once this is merged in I'll start working on doing the "pick fx order" patch. I think there is some room with the fxloop method to not only have multiple FX added, but also apply them each in their own position in the FX order.