feat(label-wrapper): Change label to be optional

It makes more sense for the label to be optional because there are no adverse effects by omitting it.

Closes #78

src/wrappers/index.js

@@ -8,7 +8,7 @@ export default ngModule => {
         template: require('./label.html'),
         apiCheck: check => ({
           templateOptions: {
-            label: check.string,
+            label: check.string.optional,
             required: check.bool.optional,
             labelSrOnly: check.bool.optional,
           }