Add optional `flushImmediate` argument to `AudioSelectionOption`

[robwalch]

flushImmediate should be added as an optional property in AudioSelectionOption, so that the default behavior can be overridden when using hls.setAudioOption to change audio tracks.

// Make a "smooth" track change to a rendition matching the audio option criteria
// (`lang`, `channels`, etc...)
hls.setAudioOption({ /* track criteria */, flushImmediate: false });