Allow every character to be present in a username.

Fix #207.

lib/src/channels/username.dart

@@ -41,7 +41,7 @@ class Username with _$Username {
       return false;
     }
 
-    return !RegExp('[^0-9a-zA-Z]').hasMatch(name);
+    return true;
   }
 
   /// Parses a username from a url.