fix(index): make replace property optional in index.d.ts

And update the return types

See #134

Author: remarkablemark

index.d.ts

@@ -7,7 +7,9 @@ import htmlToDOM from 'html-dom-parser';
 
 export interface HTMLReactParserOptions {
   // TODO: Replace `object` by type for objects like `{ type: 'h1', props: { children: 'Heading' } }`
-  replace(domNode: DomElement): React.ReactElement | object | undefined | false;
+  replace?: (
+    domNode: DomElement
+  ) => React.ReactElement | object | void | undefined | null | false;
   library?: object;
 }