Add definition for .screen-reader-text to default css

The screen-reader-text class is used more and more in WordPress. There was already a topic in the Newsmag forum complaining about this (https://forum.tagdiv.com/topic/bug-categories-label/) but nobody from the developers did answer 🙁

Please add the following to the default css and release a new Newspaper version (and BTW you could update VisualComposer as well):

.screen-reader-text {
clip: rect(1px, 1px, 1px, 1px);
position: absolute !important;
height: 1px;
width: 1px;
overflow: hidden;
}


This code is suggested by https://make.wordpress.org/accessibility/2015/02/09/hiding-text-for-screen-readers-with-wordpress-core/ and by http://blog.rrwd.nl/2015/04/04/the-screen-reader-text-class-why-and-how/ and it is used in the TwentyFifteen theme delivered with WordPress (https://core.svn.wordpress.org/trunk/wp-content/themes/twentyfifteen/style.css Section 7).

Thanks!