Feature request: Add CLI argument to specify a system prompt

I'd like to be able to set the system prompt from the call to `ollama` in my shell, rather than in the conversation. For example: ```sh ollama run llama3.1 --system="Your nickname is 'Grass' now" ``` ...or... ```sh ollama run llama3.1 -s "system" "Your nickname is 'Grass' now" ``` With this ability, I could set up aliases in my shell profile so that I can run system-prompt-customized versions of a model with a single command. ```sh alias grass="ollama run llama3.1 --system=\"Your nickname is 'Grass' now\"" ``` It'd be even better if I could also specify a path to a text file with a system prompt (e.g. `ollama run llama3.1 --system-file="~/system_prompts/grass.txt"`, but that wouldn't be necessary. This is unique from #807, whose fix only works in the conversation.