* Add _getExpressionAndReading utility
* Add _getExpressionReadingKey utility
* Add _getAudioOptions
* Separate some logic into _createAudioFromInfoList
* Update cache data format so that it can be accessed synchronously
* Allow sources to be manually specified
* Add options to enable playback of a specific entry of a source
* Use a standard map
* Wrap function
* Improve cache to support multiple different source arrays
For the same reading/expression
* Rename variable
* Use null instead of an error
* Simplify details
* Simplify audio creation
* Return an array of sources instead of a single item
* Use sourceIndex instead of index
* Rename APIs
* Return more info about the source
* Return source instead of sourceIndex
