Added preprompt

[guhitb]

Adds the value from the -preprompt cli flag before user input in each mode.
For multiline mode, the preprompt is added to the input textarea.
For interactive mode, the preprompt isn't show to the user but is still used.

Related: #151

tgptpreprompt.mp4

[aandrew-me]

You didn't add preprompt for the default mode. Here

Also, I actually wanted to use the preprompt as system prompt instead of just adding it to the prompt, since the AI would take that prompt more seriously, I think.

[aandrew-me]

Also, I just want to thank you for helping to improve the program :)

[guhitb]

Thanks for creating it! It's been really helpful for programming 😄

It looks like system prompts are implemented differently for each provider. I think I can get it working for most providers, but the opengpts format looks different. Does opengpts not have message history?

[aandrew-me]

Indeed, they are implemented differently. I can merge this request as it is, later I will individually change prompts to system prompts for each provider separately.
Opengpts changed the way they remember messages, they use thread ids with user ID in cookies.

[ssergorp]

This works for only one message using the -i, -m flags. And when you use the -m flag, you see it in the msg prompt. When I saw the -preprompt flag I assumed it would function like a config option you could set once, and from then on would automatically be used as system prompt for whatever provider or model you used.