Add platform specific configuration paths

Author: jtojnar

README.markdown

@@ -95,9 +95,13 @@ following order:
    settings)
 3. `.stylish-haskell.yaml` in the nearest ancestor directory (useful for
    per-project settings)
-4. `.stylish-haskell.yaml` in your home directory (useful for user-wide
+4. `stylish-haskell/config.yaml` in the platform’s configuration directory
+   (on Windows, it is %APPDATA%, elsewhere it defaults to `~/.config` and
+   can be overridden by the `XDG_CONFIG_HOME` environment variable;
+   useful for user-wide settings)
+5. `.stylish-haskell.yaml` in your home directory (useful for user-wide
    settings)
-5. The default settings.
+6. The default settings.
 
 Use `stylish-haskell --defaults > .stylish-haskell.yaml` to dump a
 well-documented default configuration to a file, this way you can get started

lib/Language/Haskell/Stylish/Config.hs

@@ -73,12 +73,13 @@ defaultConfigFilePath = getDataFileName "data/stylish-haskell.yaml"
 configFilePath :: Verbose -> Maybe FilePath -> IO FilePath
 configFilePath _       (Just userSpecified) = return userSpecified
 configFilePath verbose Nothing              = do
-    current  <- getCurrentDirectory
-    home     <- getHomeDirectory
-    def      <- defaultConfigFilePath
-    mbConfig <- search $
+    current     <- getCurrentDirectory
+    configPath  <- getXdgDirectory XdgConfig "stylish-haskell"
+    home        <- getHomeDirectory
+    def         <- defaultConfigFilePath
+    mbConfig    <- search $
         [d </> configFileName | d <- ancestors current] ++
-        [home </> configFileName, def]
+        [configPath </> "config.yaml", home </> configFileName, def]
 
     case mbConfig of
         Just config -> return config

stylish-haskell.cabal

@@ -125,7 +125,7 @@ Test-suite stylish-haskell-tests
     base             >= 4.8  && < 5,
     bytestring       >= 0.9  && < 0.11,
     containers       >= 0.3  && < 0.6,
-    directory        >= 1.1  && < 1.4,
+    directory        >= 1.2.3  && < 1.4,
     filepath         >= 1.1  && < 1.5,
     haskell-src-exts >= 1.18 && < 1.21,
     mtl              >= 2.0  && < 2.3,