Add support to change the output file format from synthesizeStream.

[rduke-cfpwood]

Being able to select the audio output format would be nice. or at least default it to .wav (riff-16khz-16bit-mono-pcm)

[palmerabollo]

It's a good idea @rduke-cfpwood .

The audio output format is currently hardcoded. If you feel like contributing to the project, the output format could be added as a parameter to the synthesizeStream function, keeping the current default value (for backwards compatibility) and then used as the 'X-Microsoft-OutputFormat' request header instead of hardcoding it.