Add element properties to markdown-to-jsx (#40542)

types/markdown-to-jsx/index.d.ts

@@ -8,7 +8,7 @@ import * as React from 'react';
 
 export default class Markdown extends React.Component<MarkdownProps> { }
 
-export interface MarkdownProps {
+export interface MarkdownProps extends React.HTMLAttributes<HTMLElement> {
     options?: MarkdownOptions;
     // React.ReactNode contains both null and undefined
     // tslint:disable-next-line:no-null-undefined-union

types/markdown-to-jsx/markdown-to-jsx-tests.tsx

@@ -63,3 +63,9 @@ render(
 );
 
 <Markdown options={{ slugify: str => str }}># 中文</Markdown>;
+
+// supports passing extra properties to the underlying element
+<Markdown className="markdown-body">{`
+# Header 1
+## Header 2
+`}</Markdown>;