Koenig - Add caption to HTML/embed card

refs https://github.com/TryGhost/Ghost/issues/9311
- add basic implementation of a caption field to the embed card

ghost/admin/lib/koenig-editor/addon/components/koenig-card-html.js

@@ -23,13 +23,21 @@ export default Component.extend({
 
     actions: {
         updateHtml(html) {
+            this._updatePayloadAttr('html', html);
+        },
+
+        updateCaption(caption) {
+            this._updatePayloadAttr('caption', caption);
+        }
+    },
+
+    _updatePayloadAttr(attr, value) {
         let payload = this.get('payload');
         let save = this.get('saveCard');
 
-            set(payload, 'html', html);
+        set(payload, attr, value);
 
         // update the mobiledoc and stay in edit mode
         save(payload, false);
     }
-    }
 });

ghost/admin/lib/koenig-editor/addon/templates/components/koenig-card-html.hbs

@@ -11,8 +11,20 @@
             autofocus=true
             update=(action "updateHtml")
         }}
+
+        <input
+            value={{payload.caption}}
+            type="text"
+            class="miw-100 tc bn form-text"
+            oninput={{action "updateCaption" value="target.value"}}
+            placeholder="Type caption for embed (optional)">
     {{else}}
         {{{payload.html}}}
+
+        {{#if payload.caption}}
+            <p class={{kg-style "figcaption"}}>{{{payload.caption}}}</p>
+        {{/if}}
+
         <div class="koenig-card-click-overlay"></div>
     {{/if}}
 {{/koenig-card}}