settings attribute for every class inheriting from IntervalModule are
automatically extended by interval setting if it does not exist.
A function to flatten settings in SettingsBase was extracted and added
as a static method.
Floats in mem.py can now be formatted with a standard format string
(e.g. {avail_mem:.2f}) instead of needing an extra `round` parameter.
This brings this module in line with the expected behavior in the rest
of i3pystatus.
