feat: Add default prompt via env variable

[ahernsean]

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.
• Bump version number