Add .It Xo/.Xc support

Despite some ambiguous documentation to the contrary the Xo/Xc macros
only seem genuinely useful in an .It head.

src/Text/Pandoc/Readers/Mdoc.hs

@@ -815,11 +815,14 @@ parseDefinitionList = do
   where
     dlItem = do
       macro "It"
-      dt <- listHead >>= spacify
+      dt <- (xoListHead <|> listHead) >>= spacify
       dd <- mconcat <$> many parseRegularBlock
       return (dt, [dd])
-    -- TODO support Xo/Xc
     listHead = many1 ((parseInlineMacro <|> litsAndDelimsToInlines)) <* optional eol
+    xoListHead = do
+      macro "Xo"
+      optional eol
+      many1Till parseInlines (emptyMacro "Xc")
 
 parseBl :: PandocMonad m => MdocParser m Blocks
 parseBl = do