added nix support

src/Settings.hs

@@ -33,7 +33,8 @@ data AppSettings = AppSettings
     -- ^ Configuration settings for accessing the database.
     , appRoot                   :: Maybe Text
     -- ^ Base for all generated URLs. If @Nothing@, determined
-    -- from the request headers.
+    , appSessionKey                :: [Char]
+    -- ^ Where to get the client session key
     , appHost                   :: HostPreference
     -- ^ Host/interface the server should bind to.
     , appPort                   :: Int
@@ -74,6 +75,7 @@ instance FromJSON AppSettings where
         appStaticDir              <- o .: "static-dir"
         appDatabaseConf           <- o .: "database"
         appRoot                   <- o .:? "approot"
+        appSessionKey                <- o .: "session-key"
         appHost                   <- fromString <$> o .: "host"
         appPort                   <- o .: "port"
         appIpFromHeader           <- o .: "ip-from-header"