allow tone frequency of zero #21
                
     Merged
            
            
          
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This corresponds to the issue request I placed, then decided I could make a pull request (though I'm new to the process).
This fix allows a frequency of zero specified to the play_tone function.
Issue Request: https://github.com/adafruit/Adafruit_CircuitPython_FunHouse/issues/20