Added .setVoice() to override default

[therebelrobot]

Love the library! The one thing I needed was the ability to override the default language voice, it seemed pretty restrictive before. The way I'm now using this is like:

artyom
        .initialize({
          lang: "en-US",
          continuous: true,
          debug: true, 
          listen: true,
          name: "Archimedes",
        })
        .then(() => {
          artyom.setVoice("Google UK English Female");
          // artyom.setVoice("Tessa");
          // artyom.setVoice("Veena");
        });

I found that trying to set it in the initialize function wasn't working because getVoices wasn't populated yet. putting it afterwards, though, seemed to work.

Let me know if you want more documentation or cleanup, happy to hop back in and tighten this up.