Added `valueTrue` and `valueFalse` to trait checkbox type. Closes #1424

This feature allows you to customize the boolean attribute on components
Example
```js
traits: [{
  type: 'checkbox',
  label: 'Show Name',
  name: 'show-name',
  valueTrue: 'TRUE-value',
  valueFalse: 'FALSE-value',
  // For default values you have to specify same true/false values
  // You can add `value` in trait or on your components
  value: 'FALSE-value',
}]
```

src/trait_manager/view/TraitCheckboxView.js

@@ -1,3 +1,4 @@
+import { isUndefined } from 'underscore';
 var TraitView = require('./TraitView');
 
 module.exports = TraitView.extend({
@@ -16,7 +17,23 @@ module.exports = TraitView.extend({
    * @private
    */
   onChange() {
-    this.model.set('value', this.getInputEl().checked);
+    const value = this.getInputEl().checked;
+    this.model.set('value', this.getCheckedValue(value));
+  },
+
+  getCheckedValue(checked) {
+    let result = checked;
+    const { valueTrue, valueFalse } = this.model.attributes;
+
+    if (result && !isUndefined(valueTrue)) {
+      result = valueTrue;
+    }
+
+    if (!result && !isUndefined(valueFalse)) {
+      result = valueFalse;
+    }
+
+    return result;
   },
 
   /**
@@ -29,15 +46,21 @@ module.exports = TraitView.extend({
     const el = TraitView.prototype.getInputEl.apply(this, args);
 
     if (toInit) {
-      let checked;
+      let checked, targetValue;
       const { model, target } = this;
+      const { valueTrue, valueFalse } = model.attributes;
       const name = model.get('name');
 
       if (model.get('changeProp')) {
         checked = target.get(name);
+        targetValue = checked;
       } else {
-        checked = target.get('attributes')[name];
-        checked = checked || checked === '' ? !0 : !1;
+        targetValue = target.get('attributes')[name];
+        checked = targetValue || targetValue === '' ? !0 : !1;
+      }
+
+      if (!isUndefined(valueFalse) && targetValue === valueFalse) {
+        checked = !1;
       }
 
       el.checked = checked;