Support other OpenAI endpoint

[ITsPro-CN]

It will be of great help if it supports other OpenAI endpoints.

[Crystal-Spider]

A PR would really help (I'm very short on time).

Endpoints must be added here and here. If you also find a way to allow any endpoint dynamically (without forcing specific ones, so that they are always up-to-date with OpenAI) it would be awesome.

[ITsPro-CN]

@Crystal-Spider Challenge accepted. See #35