add support for following some buttons on mobile.twitter.com

The buttons are:

  * reply
  * retweet
  * favorite
  * direct message
  * drop down menu

src/content/components/common/follow.js

@@ -4,7 +4,8 @@ import * as dom from 'shared/utils/dom';
 
 const TARGET_SELECTOR = [
   'a', 'button', 'input', 'textarea', 'area',
-  '[contenteditable=true]', '[contenteditable=""]', '[tabindex]'
+  '[contenteditable=true]', '[contenteditable=""]', '[tabindex]',
+  '[role="button"]'
 ].join(',');