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

images/svg/stackexchange.svg

@@ -0,0 +1,6 @@
+<svg
+xmlns="http://www.w3.org/2000/svg"
+aria-label="Stack Exchange" role="img"
+viewBox="0 0 512 512"><rect width="512" height="512"
+fill="#242729"
+rx="15%"/><path fill="#376db6" d="M106 232.3h298v61.4H106z"/><path fill="#4ca2da" d="M106 153h298v61.4H106z"/><path fill="#91d8f4" d="M357.4 71H153.8a48.5 48.5 0 0 0-47.8 49v15.9h298v-16c0-27-20.7-49-46.6-49z"/><path fill="#1e5397" d="M106 311.2v16c0 27 21.5 49 47.8 49h127.5V441l63-65h13.9a48.5 48.5 0 0 0 47.8-49v-16z"/></svg>