This PR adds support for having chatblade use a default prompt. The default is set via an environment variable. In some ways, it makes chatblade similar to ChatGPT's "custom instructions" that are automatically applied for all new chats.
If CHATBLADE_DEFAULT_PROMPT is set in the user's environment, use the prompt set there as if it had been specified on the command line with -p. If the user has "-p prompt" on the command line, use that instead of the environment variable.
This PR adds support for having chatblade use a default prompt. The default is set via an environment variable. In some ways, it makes chatblade similar to ChatGPT's "custom instructions" that are automatically applied for all new chats.