Add off method to unsubscribe events

Fixes mapbox#193
Inspired by tristen commit ref mapbox#40

Author: patrix

API.md

@@ -16,6 +16,7 @@
     -   [getWaypoints](#getwaypoints)
     -   [removeRoutes](#removeroutes)
     -   [on](#on)
+    -   [off](#off)
 
 ## MapboxDirections
 
@@ -168,7 +169,8 @@ Subscribe to events that happen within the plugin.
 
 **Parameters**
 
--   `type` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** name of event. Available events and the data passed into their respective event objects are:-   **clear** `{ type: } Type is one of 'origin' or 'destination'`
+-   `type` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** name of event. Available events and the data passed into their respective event objects are:
+    -   **clear** `{ type: } Type is one of 'origin' or 'destination'`
     -   **loading** `{ type: } Type is one of 'origin' or 'destination'`
     -   **profile** `{ profile } Profile is one of 'driving', 'walking', or 'cycling'`
     -   **origin** `{ feature } Fired when origin is set`
@@ -178,3 +180,21 @@ Subscribe to events that happen within the plugin.
 -   `fn` **[Function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function)** function that's called when the event is emitted.
 
 Returns **[MapboxDirections](#mapboxdirections)** this;
+
+### off
+
+Unsubscribe to events previously registered.
+
+**Parameters**
+
+-   `type` **[String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** name of event. Available events and the data passed into their respective event objects are:
+    -   **clear** `{ type: } Type is one of 'origin' or 'destination'`
+    -   **loading** `{ type: } Type is one of 'origin' or 'destination'`
+    -   **profile** `{ profile } Profile is one of 'driving', 'walking', or 'cycling'`
+    -   **origin** `{ feature } Fired when origin is set`
+    -   **destination** `{ feature } Fired when destination is set`
+    -   **route** `{ route } Fired when a route is updated`
+    -   **error** \`{ error } Error as string
+-   `fn` **[Function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function)** function used when the event was registered.
+
+Returns **[MapboxDirections](#mapboxdirections)** this;

src/actions/index.js

@@ -315,6 +315,34 @@ export function eventSubscribe(type, fn) {
   };
 }
 
+export function eventUnsubscribe(type, fn) {
+  return (dispatch, getState) => {
+    const { events } = getState();
+    
+    if (!events[type]) {
+      return {
+        type: types.EVENTS,
+        events
+      };
+    }
+    
+    if (fn) {
+      // Drop the event if it exists in the events object
+      const fnToDelete = events[type].indexOf(fn);
+      if (fnToDelete >= 0) {
+        events[type].splice(fnToDelete, 1);
+      }
+    } else {
+      delete events[type];
+    }
+
+    return {
+      type: types.EVENTS,
+      events: events
+    };
+  };
+}
+
 export function eventEmit(type, data) {
   return (dispatch, getState) => {
     const { events } = getState();

src/directions.js

@@ -551,4 +551,15 @@ export default class MapboxDirections {
     this.actions.eventSubscribe(type, fn);
     return this;
   }
+  
+  /**
+   * Unsubscribe to events
+   * @param {String} type name of event. Available events are outlined in `on`
+   * @param {Function} fn optional. The function that's called when the event is emitted.
+   * @returns {Directions} this;
+   */
+  off(type, fn) {
+    this.actions.eventUnsubscribe(type, fn);
+    return this;
+  }
 }