Lua: set "arg" instead of "PandocParameters".

This is standard for lua scripts, and I see no reason
to depart from the standard here.

Also, "arg" is now pushed onto the stack before the script
is loaded.  Previously it was not, and thus "PandocParameters"
was not available at the top level.

src/Text/Pandoc/Lua.hs

@@ -57,6 +57,8 @@ runLuaFilter datadir filterPath args pd = liftIO . Lua.runLua $ do
   -- store module in global "pandoc"
   pushPandocModule datadir
   Lua.setglobal "pandoc"
+  push args
+  Lua.setglobal "arg"
   top <- Lua.gettop
   stat <- Lua.dofile filterPath
   if stat /= OK
@@ -68,8 +70,6 @@ runLuaFilter datadir filterPath args pd = liftIO . Lua.runLua $ do
       -- Use the implicitly defined global filter if nothing was returned
       when (newtop - top < 1) pushGlobalFilter
       luaFilters <- peek (-1)
-      push args
-      Lua.setglobal "PandocParameters"
       runAll luaFilters pd
 
 pushGlobalFilter :: Lua ()