Improve autocomplete behaviour

Fixes vector-im/vector-web#1761

src/SlashCommands.js

@@ -17,6 +17,8 @@ limitations under the License.
 var MatrixClientPeg = require("./MatrixClientPeg");
 var dis = require("./dispatcher");
 var Tinter = require("./Tinter");
+import sdk from './index';
+import Modal from './Modal';
 
 
 class Command {
@@ -56,6 +58,16 @@ var success = function(promise) {
 };
 
 var commands = {
+    ddg: new Command("ddg", "<query>", function(roomId, args) {
+        const ErrorDialog = sdk.getComponent('dialogs.ErrorDialog');
+        // TODO Don't explain this away, actually show a search UI here.
+        Modal.createDialog(ErrorDialog, {
+            title: "/ddg is not a command",
+            description: "To use it, just wait for autocomplete results to load and tab through them.",
+        });
+        return success();
+    }),
+
     // Change your nickname
     nick: new Command("nick", "<display_name>", function(room_id, args) {
         if (args) {