Added custom domains you can purchase

I had a tiny bit of revenue ($12/month) via Patreon. To show my thanks I was setting users up with custom *.playit.gg domains. Now I've properly integrated stripe checkout to make it a paid feature. Here's hoping I can grow that to cover costs. I also want to expand and add more servers across the globe to lower latency. These things cost % so hopefully values align and I can continue to make the project more ambitious.