Larger character set for domain names (#11)

Domain names can include much more characters than just `a-z`, but also `A-Z`, `0-9`, and `_`.

src/Provider/GitHubProvider.php

@@ -18,7 +18,7 @@ public function fetchFromUrl(string $url): string
 
     private function getUrl(string $url): string
     {
-        $domain = preg_replace('/https?:\/\/(www\.)?([a-z\-\.]+)\/?.*/i', '$2', $url);
+        $domain = preg_replace('/https?:\/\/(www\.)?([\w\-\.]+)\/?.*/i', '$2', $url);
 
         return 'https://favicons.githubusercontent.com/'.urlencode($domain);
     }