add keepIframeSrcFn option (#81)

* add allowIframe option to snapshot

* rename allowIframe to keepIframeSrcFn

typings/types.d.ts

@@ -88,3 +88,4 @@ export declare type SlimDOMOptions = Partial<{
     headMetaVerification: boolean;
 }>;
 export declare type MaskTextFn = (text: string) => string;
+export declare type KeepIframeSrcFn = (src: string) => boolean;