Change all keyup events to keydown (#935)

For a few reasons:

- event.preventDefault() for the Enter key (to stop it from submitting
  the form when you select an item) is only effective if it's a keydown
  event.

- Using keydown for up/down navigation means you can hold them down to
  rapidly scroll through a lot of items.

- Keydown events make the UX feel more responsive, and is consistent
  with how most apps/operating systems handle key presses.

tests/unit/Dropdown.spec.js

@@ -109,14 +109,14 @@ describe("Toggling Dropdown", () => {
     expect(spy).toHaveBeenCalled();
   });
 
-  it("should remove existing search text on escape keyup", () => {
+  it("should remove existing search text on escape keydown", () => {
     const Select = selectWithProps({
       value: [{ label: "one" }],
       options: [{ label: "one" }]
     });
 
     Select.vm.search = "foo";
-    Select.find('.vs__search').trigger('keyup.esc')
+    Select.find('.vs__search').trigger('keydown.esc')
     expect(Select.vm.search).toEqual("");
   });