Lua filters: preload text module

The `text` module is preloaded in lua. The module contains some UTF-8
aware string functions, implemented in Haskell.  The module is loaded on
request only, e.g.:

    text = require 'text'
    function Str (s)
      s.text = text.upper(s.text)
      return s
    end

pandoc.cabal

@@ -327,6 +327,7 @@ library
                  scientific >= 0.2 && < 0.4,
                  vector >= 0.10 && < 0.13,
                  hslua >= 0.9 && < 0.10,
+                 hslua-module-text >= 0.1 && < 0.2,
                  binary >= 0.5 && < 0.9,
                  SHA >= 1.6 && < 1.7,
                  haddock-library >= 1.1 && < 1.5,

src/Text/Pandoc/Lua.hs

@@ -46,6 +46,7 @@ import Text.Pandoc.Lua.PandocModule (pushMediaBagModule, pushPandocModule)
 import Text.Pandoc.Lua.Filter (LuaFilter, walkMWithLuaFilter)
 import Text.Pandoc.MediaBag (MediaBag)
 import qualified Foreign.Lua as Lua
+import qualified Foreign.Lua.Module.Text as Lua
 
 runLuaFilter :: Maybe FilePath -> FilePath -> String
              -> Pandoc -> PandocIO (Either LuaException Pandoc)
@@ -64,6 +65,7 @@ runLuaFilter' :: CommonState
               -> Pandoc -> Lua Pandoc
 runLuaFilter' commonState datadir filterPath format mbRef pd = do
   Lua.openlibs
+  Lua.preloadTextModule "text"
   -- store module in global "pandoc"
   pushPandocModule datadir
   Lua.setglobal "pandoc"

stack.yaml

@@ -6,6 +6,10 @@ flags:
     network-uri: true
 packages:
 - '.'
+- location:
+    git: https://github.com/hslua/hslua-module-text
+    commit: 890ec312a91764738c4579b3a7505c8e55ecf1a5
+    extra-dep: true
 extra-deps:
 - pandoc-types-1.17.3
 - hslua-0.9.2