Allow beamer-style <...> options in raw LaTeX (also in Markdown).

This allows use of things like `\only<2,3>{my content}` in
Markdown that is going to be converted to beamer.

Closes #3184.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -423,7 +423,8 @@ inlineCommand = try $ do
   star <- option "" (string "*")
   let name' = name ++ star
   let raw = do
-        rawargs <- withRaw (skipopts *> option "" dimenarg *> many braced)
+        rawargs <- withRaw
+               (skipangles *> skipopts *> option "" dimenarg *> many braced)
         let rawcommand = '\\' : name ++ star ++ snd rawargs
         transformed <- applyMacros' rawcommand
         if transformed /= rawcommand
@@ -886,6 +887,17 @@ rawopt = do
 skipopts :: LP ()
 skipopts = skipMany rawopt
 
+-- opts in angle brackets are used in beamer
+rawangle :: LP ()
+rawangle = try $ do
+  char '<'
+  skipMany (noneOf ">")
+  char '>'
+  return ()
+
+skipangles :: LP ()
+skipangles = skipMany rawangle
+
 inlineText :: LP Inlines
 inlineText = str <$> many1 inlineChar