Add custom configuration setting

I've modified the addon to manage the custom configuration.

I've added two new options: the first enable the custom configuration and make sure that transmission start with only -g argument (all other options of addon will be ignored). The second specifies the path where is located the custom configuration. Obviously, if in the folder setted there isn't a settings.json, transmission create one ad hoc.

I've tested the new addon in my rpi with OE 4.1.2.