Added in pan level parameter

This is a simple configuration file change that allows you to specify the amount of left/right panning you get when you mix the two channels together.

Perviously all you could do was turn the mix/pan feature on and off with :pan?, like this:

      :pan? true

If you did turn panning it, it was always done with a pan level of 0.4. With this change you can
now specify the panning level:

      :pan-level 0.6

I've tried to keep the API backwards compatible, so that :pan true will still give you 0.4.