add material as field to the (Utility) Marker preset, closes #704

see https://github.com/openstreetmap/id-tagging-schema/pull/704:
> A significant distinguisher for many UK gas utility markers is colour (red vs yellow for national versus local) and material (concrete vs plastic).  I have had to add these fields to markers on probably more than on thousand occasions over the last few months while trying to edit the UK in iD, so I thought it was about time I pushed this change so that I have to type less ....

data/presets/marker.json

@@ -5,8 +5,9 @@
         "operator",
         "marker",
         "utility",
-        "inscription",
-        "colour"
+        "colour",
+        "material",
+        "inscription"
     ],
     "moreFields": [
         "height",