Add option to configure content rendering in block form

`yieldedContentRendering`:
- 'always': render always (by default, old behaviour);
- 'lazy': render since the first opening;
- 'open': render only when modal is open, content is destroyed on closing.

addon/components/ember-remodal.js

@@ -25,6 +25,8 @@ export default Component.extend({
   modifier: '',
   modal: null,
   options: null,
+  yieldedContentRendering: 'always',
+  isOpen: false,
   closeOnEscape: true,
   closeOnCancel: true,
   closeOnConfirm: true,
@@ -46,8 +48,8 @@ export default Component.extend({
   },
 
   willDestroyElement() {
-    scheduleOnce('destroy', this, '_destroyDomElements');
-    scheduleOnce('destroy', this, '_deregisterObservers');
+    this._destroyDomElements();
+    this._deregisterObservers();
   },
 
   modalId: computed('elementId', {
@@ -66,11 +68,29 @@ export default Component.extend({
     }
   }),
 
+  shouldRenderBlock: computed('yieldedContentRendering', 'isOpen', 'modal', function() {
+    let rendering = this.get('yieldedContentRendering');
+    switch (rendering) {
+      case 'always':
+        return true;
+
+      case 'lazy':
+        return !!this.get('modal');
+
+      case 'open':
+        return this.get('isOpen');
+
+      default:
+        throw new Error(`Invalid value of "yieldedContentRendering": ${rendering}`);
+    }
+  }),
+
   openDidFire: on('opened', function() {
     this.sendAction('onOpen');
   }),
 
   closeDidFire: on('closed', function() {
+    this.set('isOpen', false);
     this.sendAction('onClose');
   }),
 
@@ -209,6 +229,7 @@ export default Component.extend({
     },
 
     open() {
+      this.set('isOpen', true);
       if (this.get('modal')) {
         scheduleOnce('afterRender', this, '_openModal');
       } else {

addon/templates/components/ember-remodal.hbs

@@ -23,13 +23,15 @@
   {{/if}}
 
   {{#if hasBlock}}
-    <div class="ember-remodal inner yielded content" data-test-id="yielded">
-      {{yield (hash
-        open=(component 'ember-remodal/er-button' modalId=elementId)
-        confirm=(component 'ember-remodal/er-button' action=(action 'confirm'))
-        cancel=(component 'ember-remodal/er-button' action=(action 'cancel'))
-      )}}
-    </div>
+    {{#if shouldRenderBlock}}
+      <div class="ember-remodal inner yielded content" data-test-id="yielded">
+        {{yield (hash
+          open=(component 'ember-remodal/er-button' modalId=elementId)
+          confirm=(component 'ember-remodal/er-button' action=(action 'confirm'))
+          cancel=(component 'ember-remodal/er-button' action=(action 'cancel'))
+        )}}
+      </div>
+    {{/if}}
   {{/if}}
 
   {{#if cancelButton}}