EveryVoice

62520e19 - refactor: demo app to use callbacks maximally, making code DRYer

refactor: demo app to use callbacks maximally, making code DRYer Details: - everyvoice demo -h will now list the valid output formats - code is DRYer: we only instantiate the callbacks in synthesize_helper, which returns them so we can access them to find out the output filenames where we need them - the File Output box is hidden when wav is the only output format