Org writer: Fix output for linebreaks

Hard linebreaks in Org mode are represented by the string "\\" as the
last characters in a line.  Adds this feature to the Org-mode writer.

src/Text/Pandoc/Writers/Org.hs

@@ -271,7 +271,7 @@ inlineToOrg (Math t str) = do
               else "$$" <> text str <> "$$"
 inlineToOrg (RawInline f str) | f == "tex" || f == "latex" = return $ text str
 inlineToOrg (RawInline _ _) = return empty
-inlineToOrg (LineBreak) = return cr -- there's no line break in Org
+inlineToOrg (LineBreak) = return (text "\\\\" <> cr)
 inlineToOrg Space = return space
 inlineToOrg (Link txt (src, _)) = do
   case txt of

tests/writer.org

@@ -42,7 +42,7 @@ item.
 
 Here's one with a bullet. * criminey.
 
-There should be a hard line break
+There should be a hard line break\\
 here.
 
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