Fix disabled optgroups not being disabled in dropkick

dist/dropkick.css

@@ -123,6 +123,11 @@
     margin-bottom: 0;
     padding-bottom: 0; }
 
+.dk-optgroup-disabled {
+  opacity: 0.6;
+  color: #BBBBBB;
+  cursor: not-allowed; }
+
 .dk-optgroup-label {
   padding: 0 0.5em 0.25em;
   font-weight: bold;

src/css/dropkick.scss

@@ -167,6 +167,12 @@ $dk-disabled-color: #BBBBBB !default;
   }
 }
 
+.dk-optgroup-disabled {
+  opacity: 0.6;
+  color: $dk-disabled-color;
+  cursor: not-allowed;
+}
+
 .dk-optgroup-label {
   padding: 0 0.5em 0.25em;
   font-weight: bold;

src/dropkick.js

@@ -1115,6 +1115,15 @@ Dropkick.build = function( sel, idpre ) {
           });
 
           for ( i = node.children.length; i--; children.unshift( node.children[ i ] ) );
+
+          if (node.disabled) {
+            optgroup.classList.add('dk-optgroup-disabled');
+
+            children.forEach(option => {
+              option.disabled = node.disabled;
+            });
+          }
+
           children.forEach( addOption, optgroupList );
 
           this.appendChild( optgroup ).appendChild( optgroupList );

tests/dropkick-test.js

@@ -268,4 +268,26 @@ describe('Dropkick tests', function() {
       });
     });
   });
+
+  describe('disabled optgroups', function() {
+    beforeEach(function() {
+      $('#normal_select').append('<optgroup disabled label="Maybe"><option value="one">One</option></optgroup>');
+      this.dk.refresh();
+    });
+
+    it('has a disabled optgroup', function() {
+      expect($('.dk-optgroup').hasClass('dk-optgroup-disabled')).to.equal(true);
+    });
+
+    describe('clicking the disabled optgroup', function() {
+      beforeEach(function() {
+        this.dk.open();
+        $('.dk-optgroup-options li:first').click();
+      });
+
+      it('does not select the disabled option', function() {
+        expect($('.dk-selected ').text()).to.equal('first');
+      });
+    });
+  });
 });