Add support for captioning images in feature row helper

Use `image_caption` YAML front matter to assign a caption to the feature image, Markdown is allowed.

Close #1440

test/_pages/splash-page.md

@@ -15,6 +15,7 @@ intro:
   - excerpt: 'Nullam suscipit et nam, tellus velit pellentesque at malesuada, enim eaque. Quis nulla, netus tempor in diam gravida tincidunt, *proin faucibus* voluptate felis id sollicitudin. Centered with `type="center"`'
 feature_row:
   - image_path: assets/images/unsplash-gallery-image-1-th.jpg
+    image_caption: "Image courtesy of [Unsplash](https://unsplash.com/)"
     alt: "placeholder image 1"
     title: "Placeholder 1"
     excerpt: "This is some sample content that goes here with **Markdown** formatting."