Fix Flow type for optional parameter (react-native-image-picker#679)

Per documentation:
> The first arg is the options object for customization (it can also be null or omitted for default options)

Therefore the first parameter should be allow to be null or omitted in type definition.

Also it's not necessary to specify `callback` return type as `void`.

Author: leethree

index.js.flow

@@ -50,7 +50,11 @@ export type StorageOptions = {
     waitUntilSaved?: boolean;
 }
 
-declare export function showImagePicker(options: Options, callback: (response: Response) => void): void;
-declare export function launchCamera(options: Options, callback: (response: Response) => void): void;
-declare export function launchImageLibrary(options: Options, callback: (response: Response) => void): void;
+declare export function showImagePicker(options: ?Options, callback: (response: Response) => any): void;
+declare export function showImagePicker(callback: (response: Response) => any): void;
 
+declare export function launchCamera(options: ?Options, callback: (response: Response) => any): void;
+declare export function launchCamera(callback: (response: Response) => any): void;
+
+declare export function launchImageLibrary(options: ?Options, callback: (response: Response) => any): void;
+declare export function launchImageLibrary(callback: (response: Response) => any): void;