feat(option): to behave nicely as a child process that provides a chatbot

[grencez]

We just need a few things:

• [x] Option to not echo the priming prompt. And also get control of input before generating any tokens.
• [x] Command to insert exactly 1 line of text.
• [x] Command to generate exactly 1 line of text. It should have a byte limit and an optional prefix.

[grencez]

New example explains how to run as a coprocess: https://github.com/rendezqueue/rendezllama/tree/trunk/example/prompt/assistant_coprocess