fix: support --print-only flag (resolves #14)

[ohEmily]

Fixes #14 and includes #18 as it hasn't been merged yet but I trust it will be.

Added a slightly silly unit test ensuring that the command was added. Did manual testing in this way:

llm cmd -p "list files in current directory"              
ls -l

And ensured that the behavior for the same command without the -p or --print-only flag is unchanged.

Thank you for creating llm and a whole modular ecosystem around it, @simonw!