Add render hook for images

When rendering images from markdown, the image is now placed inside of a
figure element. The text associated with the image gets placed into a
figcaption element.

layouts/_default/_markup/render-image.html

@@ -0,0 +1,4 @@
+<figure>
+    <img src="{{ .Destination }}" alt="{{ .Text }}" {{ with .Title }} title="{{ . }}" {{ end }}/>
+    <figcaption>{{.Text | safeHTML }}</figcaption>
+</figure>