Provide directions options conveniences

[sarahsnow1]

• provide an enum for units
• provide an enum for language
  • add support for 2 and 4 letter language codes
• create separate method(s) which replace dictionary parameter and use them instead:

- (NSURLSessionDataTask * _Nullable)requestRouteWithLocations:(NSArray<OTRRoutingPoint*>* _Nonnull)locations
                     costingModel:(OTRRoutingCostingModel)costing
                    costingOption:(NSDictionary<NSString*, NSObject*>* _Nullable)costingOptions
                                   units:(OTRRoutingUnits)routingUnits
                            language:(OTRRoutingLanguage)routingLanguage
                         callback:(void (^ _Nonnull)(OTRRoutingResult  * _Nullable result,
                                                     id _Nullable invalidationToken,
                                                     NSError * _Nullable error ))callback;

Available unit and language values documented here:
https://mapzen.com/documentation/mobility/turn-by-turn/api-reference/