feat(sidenav): open all sidenavs from MdSidenavContainer

- added `open` and `close` functions that applies to both sidenavs in the container and resolves when both are finished

fixes #2591

src/lib/sidenav/sidenav.ts

@@ -367,6 +367,36 @@ export class MdSidenavContainer implements AfterContentInit {
     this._validateDrawers();
   }
 
+  /** Calls `open` of both start and end sidenavs */
+  public open() {
+    const promises: Promise<MdSidenavToggleResult>[] = [];
+
+    if (this._start) {
+      promises.push(this._start.open());
+    }
+
+    if (this._end) {
+      promises.push(this._end.open());
+    }
+
+    return Promise.all(promises);
+  }
+
+  /** Calls `close` of both start and end sidenavs */
+  public close() {
+    const promises: Promise<MdSidenavToggleResult>[] = [];
+
+    if (this._start) {
+      promises.push(this._start.close());
+    }
+
+    if (this._end) {
+      promises.push(this._end.close());
+    }
+
+    return Promise.all(promises);
+  }
+
   /**
    * Subscribes to sidenav events in order to set a class on the main container element when the
    * sidenav is open and the backdrop is visible. This ensures any overflow on the container element