Added experimental match & route to resize images to previews

It does not yet *use* the previews, it just puts them into a path with
'thumbnail' prepended.  Thanks, hakyll-gallery, for the inspiration.

blag.cabal

@@ -7,6 +7,7 @@ executable site
   main-is:          site.hs
   build-depends:    base == 4.*
                   , hakyll >= 4.6
+                  , hakyll-images >= 0.4.4
                   , filepath
                   , containers
                   , pandoc

site.hs

@@ -9,6 +9,7 @@ Portability : POSIX
 {-# LANGUAGE OverloadedStrings #-}
 import           Data.Monoid (mappend)
 import           Hakyll
+import           Hakyll.Images
 import           System.FilePath
 import           Text.Pandoc.Options
 import qualified GHC.IO.Encoding as E
@@ -61,7 +62,23 @@ main = do
     tags <- buildTags "posts/*" (fromCapture "tags/*.html")
     let postCtxTags = postCtxWithTags tags
 
-    match ("images/**" .||. "assets/**" .||. "assets_external/**" .||. "css/*" .||. "js/**") $ do
+    -- Pass through all full-size images:
+    match ("images/**") $ version "full" $ do
+        route   idRoute
+        compile copyFileCompiler
+
+    -- But also, make 640x480 reduced versions with /thumbnail
+    -- prepended to their path:
+    match ("images/**.jpg" .||. "images/**.png") $ version "thumbnail" $ do
+        route $ customRoute $ combine "./thumbnail/" . toFilePath
+        compile $ loadImage
+            >>= scaleImageCompiler 640 480
+
+    -- (I don't fully understand what role "version" plays here except
+    -- that it appears to be required for me to have two matches on
+    -- the same file paths.)
+
+    match ("assets/**" .||. "assets_external/**" .||. "css/*" .||. "js/**") $ do
         route   idRoute
         compile copyFileCompiler
 

stack.yaml

@@ -7,3 +7,4 @@ nix:
   packages: [zlib]
 
 extra-deps:
+- hakyll-images-0.4.4