add persistent option for defaults.lua

defaults.lua

@@ -1,3 +1,6 @@
+-- To make configuration changes that persists between (nightly) releases,
+-- copy defaults.lua to defaults.persistent.lua and make the changes there.
+
 -- number of page turns between full screen refresh
 -- default to do a full refresh on every 6 page turns
 DRCOUNTMAX = 6

reader.lua

@@ -7,6 +7,7 @@ freetype = require("ffi/freetype")
 Image = require("ffi/mupdfimg")
 
 require "defaults"
+pcall(dofile, "defaults.persistent.lua")
 package.path = "?.lua;common/?.lua;frontend/?.lua"
 package.cpath = "?.so;common/?.so;/usr/lib/lua/?.so"