Add a spectrogram demo

[Ravenstine]

One possible use for the Goertzel algorithm is to create a frequency spectrogram image. While this might take a lot of work, I think this is a good idea for demonstrating the possibilities.

[Ravenstine]

Is considering writing a separate spectrogram library that doesn't have audio API access built-in; you'd just throw data at it and it represents it. An existing spectrogram library could suffice, but I don't like how they all seem to directly connect to things like getUserMedia. IMO, the spectrogram should just take data but not need be involved with recording audio.