Add Google TTS Provider

[philhawthorne]

Adds a Google Translate TTS Provider, allowing users to use TTS without
an API key from an existing provider.

[jishi]

Thanks for this. However, given the way I load the different TTS providers, this would interfere with the priority order if it resides in the tts-providers folder, since I load them dynamically based and I can not guarantee the ordering of them. Meaning, that it would probably take google TTS before Microsoft or AWS for a lot of users (if not all), even though you have configuration for those other providers.

It needs to be pushed last onto the providers array in this file: https://github.com/jishi/node-sonos-http-api/blob/master/lib/helpers/try-download-tts.js in order to be just as a final attempt when no other provider has a configuration.

I should only read one level of files in my require-dir.js, so putting the google js file in a subfolder called "default" or something similar, and requiring it manually and adding it last to the providers array should suffice.

[philhawthorne]

@jishi yeah I wasn't sure exactly how that part of the script was working. Looked like it was just loading all the providers available in that folder, and adding them to an array.

Moving forward, it might be better to introduce a new config parameter "tts_provider" in settings.json. This way, instead of looping over each TTS provider every time, the system can tell which TTS provider to use.

[jishi]

Although it would be more performant, I think the overhead of empty Promises can be ignored when it simplifies the setup for the actual users. I believe that any configuration that can be omitted is a huge win from a user perspective :)