MD Reader: Smart ' after inline math

Closes #1909.

Adds new parser combinator to Parsing.hs

`a <+?> b`

:   if a succeeds, applies b and mappends
    output (if any) to result of a. If b fails,
    it's just a, if a fails, whole expression fails.

src/Text/Pandoc/Parsing.hs

@@ -161,7 +161,8 @@ module Text.Pandoc.Parsing ( anyLine,
                              setSourceColumn,
                              setSourceLine,
                              newPos,
-                             addWarning
+                             addWarning,
+                             (<+?>)
                              )
 where
 
@@ -1245,3 +1246,7 @@ addWarning mbpos msg =
 
 generalize :: (Monad m) => Parser s st a -> ParserT s st m a
 generalize m = mkPT (\ s -> (return $ (return . runIdentity) <$> runIdentity (runParsecT m s)))
+
+infixr 5 <+?>
+(<+?>) :: (Monoid a, Monad m) => ParserT s st m a -> ParserT s st m a -> ParserT s st m a
+a <+?> b = a >>= flip fmap (try b <|> return mempty) . (<>)

src/Text/Pandoc/Readers/Markdown.hs

@@ -1487,7 +1487,8 @@ code = try $ do
 
 math :: MarkdownParser Inlines
 math =  (B.displayMath <$> (mathDisplay >>= applyMacros'))
-     <|> (B.math <$> (mathInline >>= applyMacros'))
+     <|> ((B.math <$> (mathInline >>= applyMacros')) <+?>
+                            ((getOption readerSmart >>= guard) *> apostrophe <* notFollowedBy space))
 
 -- Parses material enclosed in *s, **s, _s, or __s.
 -- Designed to avoid backtracking.