Settings are inherited between modules

I have a set of local patches that fix https://github.com/enkore/i3pystatus/issues/110 https://github.com/enkore/i3pystatus/issues/121 and add callbacks. I will upload them incrementally.
This patch adds settings inheritance and set as a common setting for all module a variable enable_log that allows to allow logging on a per module basis.

For instance, one can use it (or not) to log stderr from a process launched through a callback (cf https://github.com/enkore/i3pystatus/issues/110).