with_synth options

Same as `with_fx`, I thought I was able to do this,

```
live_loop :foo do
  with_synth :dsaw, detune: [0.1, 0.4].choose do |s|
    play :e1, release: 8, cutoff: rrand(70, 130)
    sleep 8
  end
end
```

Would it be nice to add options to `with_synth` block?
