init: Start working on attachment

lib/src/giphy_attachment.dart

@@ -31,7 +31,8 @@ class GiphyAttachment extends StatelessWidget {
       );
     }
 
-    return Column(
+    return Card(
+      child: Column(
         mainAxisSize: MainAxisSize.min,
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: <Widget>[
@@ -88,6 +89,7 @@ class GiphyAttachment extends StatelessWidget {
               message: message,
             ),
         ],
+      ),
     );
   }
 }