Support multiple band pass filters

[jarmitage]

See discussion at tidalcycles/Tidal#233

It would be great to be able to do something like this in Tidal:

let eq = grp [
      cutoff_p, resonance_p,
      bandf_p, bandq_p,
      bandf_p, bandq_p,
      hcutoff_p, hresonance_p
      ]
...
let filter = "100 : 0.2 : 1000: 0.2 : 2000 : 0.2 : 5000: 0.2"
...
# eq filter

Currently you can do:

# bandf "[800, 1100, 1700, 1800]" # bandq 5