Fix type definition for functions.config() to enable code completion. (#127)

Author: jspri

src/config.ts

@@ -35,7 +35,7 @@ export function config(): config.Config {
 export namespace config {
   // Config type is usable as a object (dot notation allowed), and firebase
   // property will also code complete.
-  export type Config = any & { firebase?: firebase.AppOptions };
+  export type Config = { [key: string]: any } & { firebase?: firebase.AppOptions };
 
   /** @internal */
   export let singleton: config.Config;