Update documentation to reflect api parameter

Author: kind3r

API.md

@@ -26,6 +26,7 @@ The Directions control
 -   `options` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** 
     -   `options.styles` **[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)?** Override default layer properties of the [directions source](https://github.com/mapbox/mapbox-gl-directions/blob/master/src/directions_style.js). Documentation for each property are specified in the [Mapbox GL Style Reference](https://www.mapbox.com/mapbox-gl-style-spec/).
     -   `options.accessToken` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** Required unless `mapboxgl.accessToken` is set globally (optional, default `null`)
+    -   `options.api` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** Override default routing endpoint url
     -   `options.interactive` **[Boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)** Enable/Disable mouse or touch interactivity from the plugin (optional, default `true`)
     -   `options.profile` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** Routing profile to use. Options: `mapbox/driving-traffic`, `mapbox/driving`, `mapbox/walking`, `mapbox/cycling` (optional, default `"mapbox/driving-traffic"`)
     -   `options.alternatives` **[Boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)** Whether to enable alternatives. (optional, default `false`)

src/directions.js

@@ -22,6 +22,7 @@ import Instructions from './controls/instructions';
  * @param {Object} options
  * @param {Array} [options.styles] Override default layer properties of the [directions source](https://github.com/mapbox/mapbox-gl-directions/blob/master/src/directions_style.js). Documentation for each property are specified in the [Mapbox GL Style Reference](https://www.mapbox.com/mapbox-gl-style-spec/).
  * @param {String} [options.accessToken=null] Required unless `mapboxgl.accessToken` is set globally
+ * @param {String} [options.api="https://api.mapbox.com/directions/v5/"] Override default routing endpoint url
  * @param {Boolean} [options.interactive=true] Enable/Disable mouse or touch interactivity from the plugin
  * @param {String} [options.profile="mapbox/driving-traffic"] Routing profile to use. Options: `mapbox/driving-traffic`, `mapbox/driving`, `mapbox/walking`, `mapbox/cycling`
  * @param {Boolean} [options.alternatives=false] Whether to enable alternatives.