[feature] add command line option to send and recieve one off

[jcdietrich]

I would like to be able to do something like

❯ gemini --prompt 'Why is the sky blue?'

  That's a great question! The reason the sky appears blue is due to a
  phenomenon called Rayleigh scattering. Here's how it works:

  1. Sunlight Isn't Just White: Sunlight, while appearing white to us, is
  actually made up of all the colors of the rainbow (Red, Orange, Yellow,
  Green, Blue, Indigo, Violet - ROYGBIV). Each color has a different ...

If this output was sent to stdout then I could pipe the response and do wonderful things.

This is a lot like bash -c

❯ echo $0
zsh

❯ bash -c 'echo $0'
bash

[smemsh]

echo "are computers alive?" | gemini

seems to work, but doesn't exit the prompt on EOF. It should, and control-d alone on a line should exit the interactive shell just like it does for other shells. maybe it's a separate issue though since cli string arg might also be useful.