Configurable buffer size

[PetrGlad]

I needed to configure smaller buffer size to reduce playback delays (for a MIDI/VST application), and could not find a better way than to patch rodio. Notes:

• This is a breaking change, I needed to pass the buffer size to new_output_stream_with_format, it is likely possible to rewrite this, keeping that function intact if necessary.
• Using references to pass parameters is not strictly required, but the stream constructor does not need to own them, and it made it a bit easier to required some parameters after calling
• SampleFormat probably should be a part of StreamConfig. This will streamline the construction parameters. It is not clear to me what this affects, though.

The version from this patch works for me. I am willing to rework the request if there is a better idea how to do this.