Add support asyncio

[Accipiter7]

Since this is network dependent library it should support async / await for non-blocking access and Hence for better concurrency.
Thanks

[pndurette]

Indeed! Part of ripping out Python 2 support too..

[thedaynos]

I needed to use gtts in an async type of way so I set up a queue and a separate python script that only reads text messages from the queue and then calls gtts. That's all it does. Just loops until a message is received and then calls/plays.

[JDJGInc]

I agree with this, plus it makes work better with discord bots.

[jacksbox]

+1 would be super useful in todays async/await world