Added support for decay and gain

[grymmjack]

I added support for decay and gain.

Example code:

sound 500, 1500, "sine", 0.5, 1
sound 0, 2000
sound 1500, 1500, "sine", 0.5, 0.1
sound 0, 2000
sound 2500, 1000, "sine", 0.25, 0.05
sound 0, 2000
sound 500, 1500, "sawtooth", 0.5, 0.3
sound 0, 2000
sound 1500, 1500, "square", 0.5, 0.1
sound 0, 2000
sound 2500, 1000, "triangle", 0.25, 0.05
sound 3000, 1000
sound 0, 4000
sound 900, 400, "square", 0, 1
sound 0, 500
sound 900, 400

Default gain: 1
Default decay: 0