Persisted AudioContext for priming Web Audio engine

[compulim]

Fix #34.

Description

Before playing audio clips, on Safari, the play() function on that specific AudioContext instance, need to explicitly triggered by user gesture.

This work will enable persisted AudioContext object, or passable thru options. Developers can prime the AudioContext object by either pronouncing an empty string, or pass in a pre-primed AudioContext object.

Changelog

Breaking changes

• Instead of stopping AudioContext after all pending utterances are finished, the AudioContext is now persisted. If this is not desirable in your application and would like to control the lifetime of AudioContext object, please create your own instance and pass it as an option named audioContext when creating the ponyfill

Added

• Speech synthesis: Fix #34, in PR #36
  • Support user-controlled AudioContext object to be passed as an option named audioContext
  • If no audioContext option is passed, will create a new AudioContext object and permanently allocated
• Speech synthesis: If an empty utterance is being synthesized, will play an empty local audio clip instead, in PR #36