Implement a bot builder

As we're going to allow to change the URLs where requests are made, we need to add a new configuration option for Bot. To keep it sane, I moved advanced Bot configuration into the builder and left only the Bot::new and Bot::from_env methods. I also removed the ability to set up a proxy using the from_env! macro, because the macro doesn't scale well when there are more configuration options, and letting to set only proxies using the macro would be strange.

The support for local Bot API servers is out of scope for this MR, it will done in a later one.