Add support for prompt files (--prompt)

[kardolus]

It would be cool to have a configurable prompt that you can set either with a command line flag or an environment variable.

Right now prompts need to be passed in through a pipe. That works too but is a little less user friendly.

It would be a nice to have if the "role" can be temporarily overwritten as part of the prompt file.

role: You are tasked with summarizing build failures
prompt: When a build failure... bla