For plain list items with a single child that is a paragraph, do not wrap in paragraph html tags.

This is mimicking the behavior from org-mode's HTML exporter.

src/context/mod.rs

@@ -41,6 +41,7 @@ mod paragraph;
 mod plain_link;
 mod plain_list;
 mod plain_list_item;
+mod plain_list_simple_item;
 mod plain_text;
 mod planning;
 mod property_drawer;