Support setting an fb:app_id in settings.yml

src/Settings.hs

@@ -62,6 +62,9 @@ data AppSettings = AppSettings
     -- Example app-specific configuration values.
     , appAnalytics              :: Maybe Text
     -- ^ Google Analytics code
+    , appFacebookId          :: Maybe Int
+    -- ^ Facebook app ID, also used for analytics.
+
     , siteTitle                 :: Text
     -- ^ Site-wide title.
     , siteUsername              :: Text
@@ -96,6 +99,7 @@ instance FromJSON AppSettings where
         appSkipCombining          <- o .:? "skip-combining"   .!= defaultDev
 
         appAnalytics              <- o .:? "analytics"
+        appFacebookId             <- o .:? "fb-app-id"
         siteTitle                 <- o .: "title"
         siteUsername              <- o .: "username"