Allow to disable options with selectable function (#921)

* allow to disable options with selectable function

* add simple spec for new selectable option

* Prevent non-selectable options from being keyboard navigatable

tests/unit/Selectable.spec.js

@@ -0,0 +1,53 @@
+import { selectWithProps } from "../helpers";
+
+describe("Selectable prop", () => {
+  it("should select selectable option if clicked", () => {
+    const Select = selectWithProps({
+      options: ["one", "two", "three"],
+      selectable: (option) => option == "one"
+    });
+
+    Select.vm.$data.open = true;
+
+    Select.find(".vs__dropdown-menu li:first-child").trigger("mousedown");
+    expect(Select.vm.selectedValue).toEqual(["one"]);
+  })
+
+  it("should not select not selectable option if clicked", () => {
+    const Select = selectWithProps({
+      options: ["one", "two", "three"],
+      selectable: (option) => option == "one"
+    });
+
+    Select.vm.$data.open = true;
+
+    Select.find(".vs__dropdown-menu li:last-child").trigger("mousedown");
+    expect(Select.vm.selectedValue).toEqual([]);
+  });
+
+  it("should skip non-selectable option on down arrow keyUp", () => {
+    const Select = selectWithProps({
+      options: ["one", "two", "three"],
+      selectable: (option) => option !== "two"
+    });
+
+    Select.vm.typeAheadPointer = 1;
+
+    Select.find({ ref: "search" }).trigger("keyup.down");
+
+    expect(Select.vm.typeAheadPointer).toEqual(2);
+  })
+
+  it("should skip non-selectable option on up arrow keyUp", () => {
+    const Select = selectWithProps({
+      options: ["one", "two", "three"],
+      selectable: (option) => option !== "two"
+    });
+
+    Select.vm.typeAheadPointer = 2;
+
+    Select.find({ ref: "search" }).trigger("keyup.up");
+
+    expect(Select.vm.typeAheadPointer).toEqual(0);
+  })
+})