Allow specifying OpenAI-Organization header: (#313)

See https://platform.openai.com/docs/api-reference/introduction

> For users who belong to multiple organizations, you can pass a header
> to specify which organization is used for an API request. Usage from
> these API requests will count against the specified organization's
> subscription quota.

types/env.ts

@@ -1,4 +1,5 @@
 export interface ProcessEnv {
   OPENAI_API_KEY: string;
   OPENAI_API_HOST?: string;
+  OPENAI_ORGANIZATION?: string;
 }