Llm-gemini

- Added support for Gemini 1.5 Pro, for users with access to the API preview. Use that model with `llm -m gemini-1.5-pro-latest "prompt"`. [5](https://github.com/simonw/llm-gemini/issues/5)

- Initial prototype. [1](https://github.com/simonw/llm-gemini/issues/1)