Add Support for Routing Services for More Model

[Sponge-bink]

Would be great if ChatFred can talk to models other than ones from OpenAI, and for people who don't have access to the GPT-4 API (me, 🙃), the option is even less -- just gpt-3.5-turbo.

OpenRouter is a routing service that I have been using for weeks, providing access to model like Claude 2 from Anthropic, PaLM from Google, Llama v2 70B (recently added) from Meta, and GPT-4 from OpenAI. I believe that they are adding more.

And it's fairly simple to implement, get an API key from them (when you register, you will get about half a buck of credit for your API key to play with before you have to top-up), add two headers to the requests, change the API base URL, and you're good to go. Read the docs from them.

In fact I have been already using a modified version of ChatFred to talk to GPT-4 and Claude 2.