Replaced internal SCSS compiler with external tool

The use of libsass.dso somehow prevented GHCi to compile and run decker.
That in turn made using of intero impossible. Which is not an option.

app/decker.hs

@@ -25,7 +25,6 @@ import Text.Groom
 import qualified Text.Mustache as M ()
 import Text.Pandoc ()
 import Text.Printf ()
-import qualified Text.Sass as Sass
 import Utilities
 
 main :: IO ()
@@ -172,14 +171,7 @@ main = do
         exists <- doesFileExist src
         when exists $ do
           need [src]
-          putNormal ("# scss (for " ++ makeRelativeTo projectDir out ++ ")")
-          scss <- liftIO $ readFile src
-          result <- liftIO $ Sass.compileString scss Sass.def
-          case result of
-            Left err -> do
-              msg <- liftIO $ Sass.errorMessage err
-              throw (SassException msg)
-            Right css -> liftIO $ writeFile out css
+          sassc [src, out]
     --
     phony "clean" $ do
       removeFilesAfter publicDir ["//"]

decker.cabal

@@ -42,7 +42,6 @@ executable decker
   build-depends:       base
                      , Glob
                      , HTTP
-                     , hsass
                      , aeson
                      , ansi-terminal
                      , array

readme.md

@@ -27,6 +27,7 @@ chmod a+x decker
 
 Decker uses a few external tools that need to be installed on the system:
 
+-   [*ssh*](https://www.openssh.com) for publishing slide decks and resources
 -   [*rsync*](http://formulae.brew.sh/repos/Homebrew/homebrew-core/formula/rsync)
     for publishing slide decks and resources
 -   [*unzip*](http://formulae.brew.sh/repos/Homebrew/homebrew-core/formula/unzip)
@@ -39,13 +40,14 @@ Decker uses a few external tools that need to be installed on the system:
 -   [*Gnuplot*](http://gnuplot.sourceforge.net) to generate graphs using `dot`
 -   [*pdf2svg*](https://github.com/dawbarton/pdf2svg) to generate SVG files from
     PDF documents
+-   [*sassc*](https://github.com/sass/sassc) to compile SCSS to CSS
 
 ### Installation of external tools on macOS
 
 Use [Homebrew](https://brew.sh) to install most of them.
 
 ``` {.sh}
-brew install rsync unzip graphviz gnuplot pdf2svg
+brew install rsync unzip graphviz gnuplot pdf2svg sassc
 ```
 
 For the rest follow instructions on their respective webites.

src/External.hs

@@ -8,6 +8,7 @@ module External
   , pdflatex
   , pdf2svg
   , decktape
+  , sassc
   , checkExternalPrograms
   ) where
 
@@ -91,6 +92,14 @@ programs =
         []
         (helpText
            "Decktape PDF exporter (https://github.com/astefanutti/decktape)"))
+  , ( "sassc"
+    , ExternalProgram
+        []
+        "sassc"
+        ["--style", "nested"]
+        ["-v"]
+        (helpText
+           "Decktape PDF exporter (https://github.com/astefanutti/decktape)"))
   ]
 
 type Program = ([String] -> Action ())
@@ -119,6 +128,9 @@ pdf2svg = makeProgram "pdf2svg"
 decktape :: Program
 decktape = makeProgram "decktape"
 
+sassc :: Program
+sassc = makeProgram "sassc"
+
 helpText :: String -> String
 helpText name =
   "The " ++