Support context and storage of prompts

• CLI supports options --instruct and --context.
• Prompt relations are managed in a NetworkX graph store.
• Prompt objects are managed in a shelve database.
• Refactored code to decouple Assistant from OpenAI implementation.

Support OpenAI Chat APIs

• Focus on OpenAI chat APIs.
• Support here-doc for inputting long text as a prompt.
• Support streaming response.
• Support multiple choices in response.
• No prompt storage yet.
• No prompt history management yet.