Directly setting biquad coefficients

Chris and I were discussing the possibility of allowing a user to set the
coefficients of a biquad filter directly, as mentioned in
https://github.com/WebAudio/web-audio-api/issues/323

This ability makes it possible to have a first-order filter, which isn't
possible now. This also makes it easier to create higher-order filters from
biquads where you've decomposed the filter into a set of biquads with the
coefficients. No need to convert each biquad into a lowpass, bandpass, and
highpass filter with appropriate frrequency and Q values.

We think this is a good idea, but were not sure on exactly how to expose
this.

   - Should we have setCoefficients(b0,b1,b2,a1,a2) create a "custom"
   filter type?
      - This would preclude any kind of automation.
      - The values of frequency, Q, gain would be undefined (in some way)
      for this filter type.
   - Should the individual coefficients be exposed as audioparams?
   - We couldn't come up with an actual use-case where any one would want,
      say, a linear ramp for, say, coefficient a1
      - Some possibly interesting filtering affects might be possible.

Thoughts?

--
Ray