More elaborate fetcher API's

src/fetchers/fetch.ts

@@ -11,12 +11,17 @@ export type FetchOps = {
 
 export type FetchHeaders = {
   get(key: string): string | null;
+  set(key: string, value: string): void;
 };
 
 export type FetchReply = {
   text(): Promise<string>;
   json(): Promise<any>;
+  extraHeaders?: FetchHeaders;
+  extraUrl?: string;
   headers: FetchHeaders;
+  url: string;
+  status: number;
 };
 
 export type FetchLike = (url: string, ops?: FetchOps | undefined) => Promise<FetchReply>;