Add colored noise wave to 'wtosc'

[olofson]

Add a noise generator where the 'p' register controls tilts the frequency spectrum of the noise. 0 would result in white noise, 1.0 would result in violet noise (+6 dB/oct), -1.0 would result in brown noise (-6 dB/oct) etc. Or maybe just interpret 'p' as dB/oct? Not quite sure what's most logical/useful there, form a musical POV, but I'm leaning towards "when in doubt, use widely known units."

See #300 for background and details.