Fixed regression in parsing _emph_

There was a bug in parsing '_emph_, ...':  when followed by
a comma, underscore emphasis did not register.  (Thanks to
gwern for pointing this out.)

This bug was introduced by the change in
c66921f2ac

src/Text/Pandoc/Readers/Markdown.hs

@@ -1059,7 +1059,7 @@ strChar = noneOf (specialChars ++ " \t\n")
 str :: GenParser Char ParserState Inline
 str = do
   a <- strChar
-  as <- many (strChar <|> (try $ char '_' >>~ lookAhead strChar))
+  as <- many (strChar <|> (try $ char '_' >>~ lookAhead alphaNum))
   let result = a:as
   state <- getState
   let spacesToNbr = map (\c -> if c == ' ' then '\160' else c)