Add support for reading `OPENAI_API_KEY` from a file path specified in the config

[giladbarnea]

My sgpt config, including the .sgptrc file, is backed up via source control. This means I constantly need to manually make sure the OPENAI_API_KEY line is kept out of commits.

This is cumbersome and only takes one lapse in concentration to accidentally push my key.

A simple fix would be to let OPENAI_API_KEY point to a file path. This separate file would hold the key in plaintext, making it easier to keep out of version control.

Corresponding PR: #589