Add thread count customization 🔥
**Is your feature request related to a problem? Please describe.** 🤔
It would be great to be able to change the amount of threads that the web server is running with, when, for example, using httpbeast. Running everything in 1 thread isn't ideal, as, for example, some database operations can take time, slowing down the whole site.

**Describe the solution you'd like** 💡
An additional optional argument could be added in serve() for setting the amount of threads that the web server will run on.