Lua: allow returning a single filter from filter files

It is now possible to return a single filter from a filter file, e.g.

``` lua
-- Switch single- and double quotes
return {
  Quoted = function (q)
    elem.quotetype = elem.quotetype == 'SingleQuote'
      and 'DoubleQuote' or 'SingleQuote'
    return elem
  end
}

The filter must not contain numerical indexes, or it might be treated as
a list of filters.

pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs

@@ -43,7 +43,10 @@ runFilterFile' envIdx filterPath doc = do
       -- filter if nothing was returned.
       luaFilters <- forcePeek $
         if newtop - oldtop >= 1
-        then peekList peekFilter top      -- get from explicit filter table
+        then liftLua (rawlen top) >>= \case
+          -- explicitly returned filter, either a single one or a list
+          0 -> (:[]) <$!> peekFilter top  -- single filter
+          _ -> peekList peekFilter top    -- list of explicit filters
         else (:[]) <$!> peekFilter envIdx -- get the implicit filter in _ENV
       settop oldtop
       runAll luaFilters doc

pandoc-lua-engine/test/lua/single-to-double-quoted.lua

@@ -1,10 +1,8 @@
 return {
-  {
     Quoted = function (elem)
       if elem.quotetype == "SingleQuote" then
         elem.quotetype = "DoubleQuote"
       end
       return elem
     end,
-  }
 }