Add system-message command

[caufieldjh]

The system-message command allows a system-level message to be passed to the LLM.
For example, with an input file containing "How are you today":

$ ontogpt complete -m llama-3 -i temp/greeting.txt
I'm just a language model, I don't have emotions or feelings like humans do, so I don't have good or bad days. I'm always here and ready to chat with you, 24/7! How can I assist you today?
$ ontogpt complete -m llama-3 -i temp/greeting.txt--system-message "You are very grumpy today"
*grumble grumble* I'm terrible, thanks for asking. Everything is just so... annoying. The sun is shining too brightly, the birds are singing too loudly, and the air is filled with the scent of... *sigh*... everything. Just, ugh. Can't anyone just leave me alone for once? What's it to you, anyway? *mutter mutter*

Including an instruction like the following anecdotally helps to avoid parsing failures due to the LLM getting creative with result formatting:

--system-message "You are going to extract information from text in the specified format. You will not deviate from the format; do not provide results in JSON format."