Expand shield coverage beyond the United States

Nobody has claimed this yet.

The turn banner and step table show graphical route shields for a number of route networks in the United States. Shields.plist maps `ref` prefixes to file name patterns on Wikimedia Commons. Commons has a much larger repertoire of shields, but we’ve run up against the limits of parsing the `ref` tag on ways, and even what we have now is prone to false positives. For example:

* [`CA`](https://github.com/mapbox/mapbox-navigation-ios/blob/1677c6479a2a293172a4a4d1e3ea45e5c1e3707b/MapboxNavigation/Resources/Shields.plist#L29-L30) may denote California or Cantabría, Spain
* [`NH`](https://github.com/mapbox/mapbox-navigation-ios/blob/1677c6479a2a293172a4a4d1e3ea45e5c1e3707b/MapboxNavigation/Resources/Shields.plist#L83-L84) may denote New Hampshire or National Highway (e.g., in India)
* [`CO`](https://github.com/mapbox/mapbox-navigation-ios/blob/1677c6479a2a293172a4a4d1e3ea45e5c1e3707b/MapboxNavigation/Resources/Shields.plist#L31-L32) may denote Colorado or a county road.
* We can’t add support for German shields to the turn banner, because we can’t tell whether to use a German, Austrian, Australian, or Chinese shield for [`A`](https://en.wikipedia.org/wiki/A_road).

Unfortunately, there isn’t a better way to distinguish shields internationally until either Project-OSRM/osrm-backend#482 or Project-OSRM/osrm-backend#333 is implemented. This issue affects routes in the U.S. as well: state trunkline highways in Michigan have the prefix `M`, which is identical to motorways in several other countries.

For more background on graphical route shields and why they matter in navigation, see gravitystorm/openstreetmap-carto#508 and [this OSM diary entry](http://www.openstreetmap.org/user/Minh%20Nguyen/diary/39123), or watch [this talk from State of the Map 2011](https://wiki.openstreetmap.org/wiki/SotM_2011_session:_Shields_Up!).

/ref https://github.com/mapbox/mapbox-navigation-ios/issues/286#issuecomment-309649781
/cc @ericrwolfe @danpat @freenerd

Start with MapboxNavigation/Resources/Shields.plist and the turn banner and step table code that consumes it. Review the referenced OSRM issues #482 and #333 to determine whether the required route-network information is available. The issue is done only when shields can be expanded internationally without the documented false positives and with consistent turn-banner behavior.

Written by the indexing model from the issue text.