Add env OPENAI_API_URL for other OpenAI compatible api url

[yurenchen000]

Try to support feature mentioned at #1

Usage:

$ time \
  OPENAI_MODEL='deepseek-chat' \
  OPENAI_API_KEY='YOUR DEEPSEEK API_KEY HERE' \
  OPENAI_API_URL='https://api.deepseek.com/v1/chat/completions'  \
  ./howto  find files modified today 

real	0m4.977s

it works,
but response slow..