Fixed latex macro parsing.

Now latex macro definitions are preserved when output is latex,
and applied when it is another format, as originally intended.

Partially addresses #730.
\providecommand is still not supported.  For this we need changes
to texmath.

src/Text/Pandoc/Parsing.hs

@@ -148,7 +148,7 @@ where
 
 import Text.Pandoc.Definition
 import Text.Pandoc.Options
-import Text.Pandoc.Builder (Blocks, Inlines)
+import Text.Pandoc.Builder (Blocks, Inlines, rawBlock)
 import qualified Text.Pandoc.UTF8 as UTF8 (putStrLn)
 import Text.Parsec
 import Text.Parsec.Pos (newPos)
@@ -1007,7 +1007,7 @@ nested p = do
 --
 
 -- | Parse a \newcommand or \renewcommand macro definition.
-macro :: Parser [Char] ParserState Block
+macro :: Parser [Char] ParserState Blocks
 macro = do
   apply <- getOption readerApplyMacros
   inp <- getInput
@@ -1018,8 +1018,8 @@ macro = do
                            then do
                              updateState $ \st ->
                                st { stateMacros = ms ++ stateMacros st }
-                             return Null
-                           else return $ RawBlock "latex" def'
+                             return mempty
+                           else return $ rawBlock "latex" def'
 
 -- | Apply current macros to string.
 applyMacros' :: String -> Parser [Char] ParserState String

src/Text/Pandoc/Readers/LaTeX.hs

@@ -204,7 +204,7 @@ block :: LP Blocks
 block = (mempty <$ comment)
     <|> (mempty <$ ((spaceChar <|> newline) *> spaces))
     <|> environment
-    <|> mempty <$ macro
+    <|> macro
     <|> blockCommand
     <|> paragraph
     <|> grouped block

src/Text/Pandoc/Readers/Markdown.hs

@@ -355,7 +355,7 @@ parseBlocks = mconcat <$> manyTill block eof
 
 block :: MarkdownParser (F Blocks)
 block = choice [ codeBlockFenced
-               , guardEnabled Ext_latex_macros *> (mempty <$ macro)
+               , guardEnabled Ext_latex_macros *> (macro >>= return . return)
                , header
                , lhsCodeBlock
                , rawTeXBlock