Support beamer \alert in LaTeX reader. Closes #4091.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1245,6 +1245,7 @@ inlineCommands = M.union inlineLanguageCommands $ M.fromList $
   , ("textup", extractSpaces (spanWith ("",["upright"],[])) <$> tok)
   , ("texttt", ttfamily)
   , ("sout", extractSpaces strikeout <$> tok)
+  , ("alert", skipangles >> spanWith ("",["alert"],[]) <$> tok) -- beamer
   , ("lq", return (str "‘"))
   , ("rq", return (str "’"))
   , ("textquoteleft", return (str "‘"))

test/command/4091.md

@@ -0,0 +1,6 @@
+```
+% pandoc -f latex
+\alert<3>{foo}
+^D
+<p><span class="alert">foo</span></p>
+```