Center images when they are the only contents in a paragraph.

src/context/paragraph.rs

@@ -12,6 +12,7 @@ use super::RenderObject;
 #[serde(rename = "paragraph")]
 pub(crate) struct RenderParagraph {
     children: Vec<RenderObject>,
+    is_single_image: bool,
     post_blank: organic::types::PostBlank,
 }
 
@@ -26,6 +27,7 @@ render!(RenderParagraph, IParagraph, original, render_context, {
 
     Ok(RenderParagraph {
         children,
+        is_single_image: original.is_single_image(),
         post_blank: original.post_blank,
     })
 });