Basic destination sign handling.

This first part in our Exit Sign / Destination Sign quest re-wires
the `destination=` and `destination:ref=` tag to the `ref=` tag (in case
it's the highway does not already have a `ref=` tag).

Doing some analysis on both Berlin and San Francisco it looks like this
will add a couple of thousand `ref=` tags that we will announce in guidance.

References:
- https://github.com/Project-OSRM/osrm-backend/issues/2267#issuecomment-214771682
- http://wiki.openstreetmap.org/wiki/Key:destination
- http://wiki.openstreetmap.org/wiki/Proposed_features/Destination_details

taginfo.json

@@ -204,6 +204,16 @@
             "object_types": [ "way" ],
             "description": "Ref of road for navigation instructions, overrides name."
         },
+        {
+            "key": "destination",
+            "object_types": [ "way" ],
+            "description": "Destination of road for navigation instructions, supplements name."
+        },
+        {
+            "key": "destination:ref",
+            "object_types": [ "way" ],
+            "description": "Destination of road for navigation instructions, supplements name."
+        },
         {
             "key": "junction",
             "object_types": [ "way" ],