runningMin in trig

Track maximum level.  Outputs the maximum value received at the
input.  When triggered, the maximum output value is reset to the
current value.

in   - input signal
trig - reset the output value to the current input value

> do { n <- dust AR 20
>    ; let { t = impulse AR 0.4 0
>          ; f = runningMin n t * 500 + 200 }
>      in audition (out 0 (sinOsc AR f 0 * 0.2)) }

> let { o = sinOsc KR 2 0
>     ; x = mouseX KR 0.01 10 Exponential 0.1
>     ; t = impulse AR x 0 
>     ; f = runningMin o t * 500 + 200 }
> in audition (out 0 (sinOsc AR f 0 * 0.2))