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/reference/stackexchange.svg

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 viewBox="0 0 120 120" style="enable-background:new 0 0 120 120;" xml:space="preserve">
+<style type="text/css">
+	.st0{fill:#376DB6;}
+	.st1{fill:#4CA2DA;}
+	.st2{fill:#91D8F4;}
+	.st3{fill:#1E5397;}
+</style>
+<rect x="22.4" y="57.5" class="st0" width="74.8" height="15.4"/>
+<rect x="22.4" y="37.6" class="st1" width="74.8" height="15.4"/>
+<path class="st2" d="M85.5,17H34.4c-6.6,0-12,5.5-12,12.3v4h74.8v-4C97.2,22.5,92,17,85.5,17z"/>
+<path class="st3" d="M22.4,77.3v4c0,6.8,5.4,12.3,12,12.3h32v16.3l15.8-16.3h3.5c6.6,0,12-5.5,12-12.3v-4H22.4z"/>
+</svg>