Add a command line option to hide kernel threads

This adds a command line option to ignore kernel threads in process list, which in my opinion clutter the display.
This only applies to Unix systems.
A nice side effect is that the CPU usage of Glances goes down a bit (-0.5 et -1% on my machine, as measured by Glances of course).
We should also maybe consider making this the default, as the kernel task on Mac is already ignored.