Add print styles

Author: Malvoz

src/mapml.css

@@ -446,4 +446,13 @@ summary {
 .mapml-layer-remove-button {
   margin-left: 15px;
   text-decoration: none;
-}
+}
+ 
+ /* Force printers to include background-images of controls for printing.
+   (https://github.com/Maps4HTML/Web-Map-Custom-Element/issues/294) */
+@media print {
+  .leaflet-control {
+    -webkit-print-color-adjust: exact;
+                  color-adjust: exact;
+  }
+}