V3 - Remove mutable class properties plus other misc changes (#781)

* Remove the mutableValue prop in the Select component.

* Add back mutable value when Vue Select has to manage its own value.

* Remove mutableOptions, valueAsAarray. Update webpack minifer to use Terser.

* Fix tabbing

* Fix bug with showClearButton

* Fix tests.

* Call clearSelection when possible

* Update dev sandbox to have all three options for setting value.

* Update dev sandbox to display current value

* Remove unused karma test setup.

* Revert onInput name change.

* Use coveralls

* Change this.internalValue to this.$data._value.

* Remove onInput prop and replace with internal method, updateValue.

* Update tests.

* Rename optionObjectComparator to optionComparator.

tests/unit/Labels.spec.js

@@ -36,7 +36,7 @@ describe("Labels", () => {
     });
 
     expect(Select.vm.searchPlaceholder).toEqual("foo");
-    Select.vm.mutableValue = "one";
+    Select.vm.$data._value = "one";
     expect(Select.vm.searchPlaceholder).not.toBeDefined();
   });
 });