RST reader: better handling for headers with an anchor.

Instead of creating a div containing the header, we put
the id directly on the header. This way header promotion
will work properly. Closes #4240.

test/command/4240.md

@@ -0,0 +1,33 @@
+```
+% pandoc -f rst -s -t native
+=====
+Title
+=====
+
+--------
+Subtitle
+--------
+
+header1
+=======
+
+header2
+-------
+
+.. _id:
+
+header3
+~~~~~~~
+
+.. _id2:
+.. _id3:
+
+header4
+~~~~~~~
+^D
+Pandoc (Meta {unMeta = fromList [("subtitle",MetaInlines [Str "Subtitle"]),("title",MetaInlines [Str "Title"])]})
+[Header 1 ("header1",[],[]) [Str "header1"]
+,Header 2 ("header2",[],[]) [Str "header2"]
+,Header 3 ("id",[],[]) [Str "header3"]
+,Header 3 ("id3",[],[]) [Str "header4",Span ("id2",[],[]) []]]
+```