BatteryStat plugin and instruction of KoboLight plugin (#2643)

* Start battery stat plugin

* BatteryStat & kobolight

* Several minor improvements

* Remove a useless function

* flush settings

* Some review feedbacks

* Resolve review comments

* Remaining Minutes -> Remaining Hours

* Add dump_file

* typo

* realpath

* realpath on folder

* Remove useless os.time()

* Resolve review comments

* warning

* Add BatteryStat.debugging flag

* treat log as txt

* Minor improvement

* Charging hour should be positive

* Use warn instead of info

* onSuspend in Kobo

* Charging events for kobo and kindle

* More events

* dumpOrLog

* Warnings

* Typo

* More space

* Singleton

* slightly format change

* BatteryStat singleton

* Init

* Remove debugging flag

* sleeping percentage is still negative

* Read settings

* Do not need to change was_suspending and was_charging

* Typo

* Remove debugging flag

* Not charging should happen before suspend

* Resolve review comments

* was_suspend and was_charging should be updated each time in onCallback()

frontend/pluginloader.lua

@@ -22,13 +22,14 @@ function PluginLoader:loadPlugins()
             local ok, plugin_module = pcall(dofile, mainfile)
             if not ok or not plugin_module then
                 logger.warn("Error when loading", mainfile, plugin_module)
-            end
-            if ok and plugin_module and not plugin_module.disabled then
+            elseif type(plugin_module.disabled) ~= "boolean" or not plugin_module.disabled then
                 package.path = package_path
                 package.cpath = package_cpath
                 plugin_module.path = path
                 plugin_module.name = plugin_module.name or path:match("/(.-)%.koplugin")
                 table.insert(self.plugins, plugin_module)
+            else
+                logger.info("Plugin ", mainfile, " has been disabled.")
             end
         end
     end