Add date metadata

Not yet exported as a coloring (year is preferable, see previous commit)
but this allows the sampling date to appear when clicking on tips and
as a filtering option.

phylogenetic/defaults/auspice_config.json

@@ -65,6 +65,7 @@
     "author",
     "strain",
     "division",
-    "location"
+    "location",
+    "date"
   ]
 }