Add intent property to Button component (#1384)

Author: Yuriko Takahashi

src/buttons/src/Button.js

@@ -136,6 +136,11 @@ Button.propTypes = {
    */
   ...layout.propTypes,
 
+  /**
+   * The intent of the button.
+   */
+  intent: PropTypes.string,
+
   /**
    * The appearance of the button.
    */