Create a new gptme.toml option `context_cmd` to support context-generating scripts

[ErikBjare]

Create a new gptme.toml option prompt_command/context_cmd which generates context on startup (instead of run.sh magic)

This would essentially remove the need for run.sh at all: https://github.com/gptme/gptme-agent-template/blob/master/run.sh