Add common environment variables

[dustinblackman]

Overview

Following up on #6, add common environment variables shared across other apps for OpenAI such as:

• OPENAI_MODEL
• OPENAI_API_KEY
• OPENAI_KEY

[suhr]

It would be nice to support OLLAMA_URL as well.

[dustinblackman]

While I see the appeal of this, Oatmeal's environment variable configs are tied to clap, which only allows one environment variable. Decoupling that and having to mange several variants of envs sounds tedious. The following I'd say is good enough.

export OPENAI_KEY=keysecretkey
export OATMEAL_OPENAI_TOKEN=$OPENAI_KEY

Marking as wont fix.