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

profiles/lib/destination.lua

@@ -0,0 +1,27 @@
+local Destination = {}
+
+function Destination.get_destination(way)
+  local destination = way:get_value_by_key("destination")
+  local destination_ref = way:get_value_by_key("destination:ref")
+
+  -- Assemble destination as: "A59: Düsseldorf, Köln"
+  --          destination:ref  ^    ^  destination
+
+  local rv = ""
+
+  if destination_ref and destination_ref ~= "" then
+    rv = rv .. destination_ref
+  end
+
+  if destination and destination ~= "" then
+      if rv ~= "" then
+          rv = rv .. ": "
+      end
+
+      rv = rv .. string.gsub(destination, ";", ", ")
+  end
+
+  return rv
+end
+
+return Destination