Add database to config

Add the database to the config file so we can override it for
unittests.

src/lua/config.lua.in

@@ -5,5 +5,6 @@ A one-stop-shop for runtime configuration
 return {
 	domain = "<{get domain}>",
 	production = false,
-	legacy_url_cutoff = 144
+	legacy_url_cutoff = 144,
+	db = "data/posts.db"
 }