Add promise to pending request

[Syed-Umair]

It would be helpful if I was able to track my request in promise
Resolve after all the requests sent to google have come back.

[bcomnes]

The underlying google voice recognition api that this depends upon is evented, so to return a promise would require some simplifying assumptions, such as single utterance.

If you come up with a connivence wrapper that has this API I'll consider a PR.

If you are talking about actual network requests you will have to dig into the underlying modules as this only deals with the the API that google provides with their voice recognition node module and doesn't have control over that.