Add Stack Exchange (#147)

* Stack Exchange: add ref

from https://stackoverflow.com/company/logos

* Stack Exchange: add STI

Ref. file from https://stackoverflow.com/company/logos
Background color from the https://stackexchange.com/ title bar (.top-bar)
Minified using https://jakearchibald.github.io/svgomg/

* Remove class

* Add ref

* Add radius

* Compliant layout

https://github.com/edent/SuperTinyIcons/blob/master/CONTRIBUTING.md

radius.html

@@ -83,7 +83,7 @@
 			'plex',       'slack',       'xing',       'pinboard',   'internet_archive', 'samsunginternet',
 			'baidu',      'twitch',      'ok',         'pocket',     'stumbleupon',  'viber',
 			'workato',    'sketch',      'vlc',        'google_maps','access',       'ethereum',
-			'opencast'
+			'opencast', 'stackexchange'
 		 ];
 
 		var container = document.querySelector('.icons');