Add add_prompt_template method to TemboRAG class

[EvanHStanton]

Bump version

• Bump version from 0.2.0 to 0.3.0

Update rag.py

• Add add_prompt_template method to TemboRAG class
• This function allows for adding custom prompts to vectorize.prompts

Update test_rag.py

• Add test_add_prompt_template to test functionality of new method

Update Makefile

• Add run.postgres to test command
• Update run.postgrescommand to run create extension vectorize cascade;

Update README.md

• Add section: Adding Custom Prompts