Llm-openrouter

- Added missing `httpx` dependency, thanks [spinagon](https://github.com/spinagon). [#4](https://github.com/simonw/llm-openrouter/issues/4)
- Now respects `OPENROUTER_KEY` environment variable, thanks [Chris Mungall](https://github.com/cmungall). [#7](https://github.com/simonw/llm-openrouter/issues/7)
- Fixed error that occurred if `urllib3` was not installed. [9](https://github.com/simonw/llm-openrouter/issues/9)

- Initial release. Adds support for models hosted by [openrouter.ai](https://openrouter.ai/). [#1](https://github.com/simonw/llm-openrouter/issues/1)