Add command line arguments

[MarkAHarley]

Some variables should be defined in the command line to make them easily configurable by the user. These include:

• ALSA input and output handles
• Microphone gain
• Number of filters

All of these should have default values. Check if c++ has a modern version of command line args other than main(argv[])....

[MHCooke]

I wonder if a whole config file would be a better idea. We have quite a lot of magic numbers and it would be good to have them in one place where they can be edited without recompilation.

[MHCooke]

Magic numbers:

• Input device name
• Output device name
• number of filters
• number of samples in rms history
• thread timeout
• vocoder filter frequency range