Support OpenRouter/alternate urls and arbitrary models

[hlidotbe]

Hi,

While we can use OPENAI_BASE_URL to use an alternative OpenAI compatible service it would be easier to be able to provide this in the config instead.

In the same spirit, opening the list of models would allow using the free/better ones available on OpenRouter or elsewhere.

I'll try a PR later but I don't now much about python so it'll probably not be very good.