Twitch Support!

Added Twitch.tv support!

WARNING: In my testing, I got temporarily IP-banned pretty fast, so don't check Twitch names too often using this checker until I add proxy support.

In other news, I added some much needed installation instructions. They don't go too in depth, but it should be enough to get you up and running quickly.

If you haven't already, please consider starring the repository to help my tool get noticed!