* (experiment) consider reducing IF filter bandwidth to ~ 80 kHz
* (experiment) consider downsampling IF signal before FM detection

* (feature) support 'M' 'k' suffixes for sample rates and tuning frequency
* (feature) implement stereo pilot pulse-per-second
* (speedup) maybe replace high-order FIR downsampling filter with 2nd order butterworth followed by lower order FIR filter
* figure out why we sometimes lose stereo lock
* (feature) implement RDS decoding
* (quality) consider FM demodulation with PLL instead of phase discriminator