Support lower sample rates for AudioPlaySerialflashRaw

[floretan]

There is already an example for support for sample rates of 11.025 and 22.05 kHz in https://github.com/PaulStoffregen/Audio/blob/master/play_memory.cpp, it would be nice to add the same functionality to AudioPlaySerialflashRaw, so more data can be stored on the external memory (quite practical if you have many samples, especially if they are long).

The AudioPlayMemory implementation reads the format (sample rate & ulaw compression) from the first element in the array, that information is not present in raw files. Would it be ok it to add an optional parameter to AudioPlaySerialflashRaw->play() to specify the format (default is 44.1 kHz without ulaw), using the same values as AudioPlayMemory?

[thebiguno]

I have just made a change to PlaySerialFlashRaw to support u-law encoding (see pull request #160); the state variables are now ready to add more options. Currently it uses the filename extension to detect whether the file is raw (.RAW) or u-law (.ULW), so we would have to either come up with other file extensions for other bit rates / encodings, or muck with the play function as you suggested.