Configurable distance units

Usage example:
    L.mapbox.directions({units: 'metric'});

The default remains imperial units.

Fixes #75

src/directions.js

@@ -7,6 +7,10 @@ var request = require('./request'),
 var Directions = L.Class.extend({
     includes: [L.Mixin.Events],
 
+    options: {
+        units: 'imperial'
+    },
+
     statics: {
         URL_TEMPLATE: 'https://api.tiles.mapbox.com/v4/directions/{profile}/{waypoints}.json?instructions=html&geometry=polyline&access_token={token}',
         GEOCODER_TEMPLATE: 'https://api.tiles.mapbox.com/v4/geocode/mapbox.places/{query}.json?proximity={proximity}&access_token={token}'

src/instructions_control.js

@@ -41,7 +41,9 @@ module.exports = function (container, directions) {
 
         steps.append('div')
             .attr('class', 'mapbox-directions-step-distance')
-            .text(function (step) { return step.distance ? format.imperial(step.distance) : ''; });
+            .text(function (step) {
+                return step.distance ? format[directions.options.units](step.distance) : '';
+            });
 
         steps.on('mouseover', function (step) {
             directions.highlightStep(step);

src/routes_control.js

@@ -37,7 +37,9 @@ module.exports = function (container, directions) {
 
         routes.append('div')
             .attr('class', 'mapbox-directions-route-details')
-            .text(function (route) { return format.imperial(route.distance) + ', ' + format.duration(route.duration); });
+            .text(function (route) {
+                return format[directions.options.units](route.distance) + ', ' + format.duration(route.duration);
+            });
 
         routes.on('mouseover', function (route) {
             directions.highlightRoute(route);