[opus] low pass filter frequency adjustable

[Mike Kaplan]

Hi, recently I made some test with the opus tools (enc and dec) and I'm 
very (and positively) surprised about the resultant quality. But the 
only think that I miss is the ability to change the low pass filter 
frequency via "--lowpass" option or similar. For example at a quality or 
96 kbps the cut off of the filter starts at 16Khz and is completely cut 
at 20 Khz. But in case of testing or adjusting streams for special 
purposes would be good the possibility of change the frequency both 
above 20Khz and below 20Khz (or other).

I guess that the cut-off frequency is specified somewhere in the code, 
so should not be hard to add this optionvia command line. I hope that 
the developers take this into account in future versions.

---
Mike