A function to use default prompts in the TOML config file

[shuheykoyama]

Assumed User Scenarios

• If the user doesn't have an OpenAI API Key (doesn't generate images), uses the sample images for imgcat or ASCII art.
• If the user has an OpenAI API Key but doesn't generate images (doesn't create prompts), uses the prompts in config.toml with options.
• If the user has an OpenAI API Key and wants to generate images, enter the prompt as an option.

[shuheykoyama]

Usage ideas

• dgen # default screen saver (color image of dgen chan...?)
• dgen -a # default screen saver (ASCII art of dgen chan...?)
• dgen girl # use prompt corresponding to 'girl' in config.toml (same for ASCII art)
• dgen cat # use prompt corresponding to 'cat' in config.toml (same for ASCII art)
• dgen -p <PROMPT> # pass your own prompt