Make the domain for the https upgrade proxy configurable

[sooophies]

Currently unclear how to configure the domain of the https upgrade proxy.
Need to find a way to pass the configuration file to this part of the code without reorganizing the project structure too much.

https://github.com/typable/verbal/blob/6092057c74e4302d739bcd677de15b80741c3b8a/src/utils.rs#L13

[sooophies]

Method was removed in pull request #4. Needs to be implemented again.

[sooophies]

A overall solution how to detect upgradable URLs is discussed in #15.