remove -1 default parameters

[telephon]

Currently, we have this:

delayfeedback :: Pattern Double -> ParamPattern
delayfeedback = make' VF delayfeedback_p
delayfeedback_p = F "delayfeedback" (Just (-1))

Which will send -1 instead of nothing. Assuming that delay is the "keyword" that will say that you want to actually use the delay, there seems to be no reason why we shouldn't just send nil (aka Nothing) instead.

The current behaviour requires special treatment of the delaytime and delayfeedback parameters.

[lennart]

see #152 for reasons why we keep these defaults.