Add checkbox demo

Author: austinkelleher

components/app-checkbox/.demo.marko.swp

@@ -1,5 +1,17 @@
+class {
+  onInput() {
+    this.state = { checked: true };
+  }
+
+  handleCheckboxToggle() {
+	  this.state.checked = !this.state.checked;
+  }
+}
+
+<h1>Is checked: ${state.checked}</h1>
 <app-checkbox
   label="Foo"
   data={name: 'foo'}
-  checked=state.checked.foo
+  checked=state.checked
   onToggle("handleCheckboxToggle")/>
+