Introduced separate load module

The module provides the three system load averages. This is an
improvement compared what you can do with the cpu module: cpu
only provides the one minute sample and the state of the cpu module is
derived from the cpu usage which messes up the formating of the load
average. Also, at least on modern Linux systems, the load of a system
takes much more than the cpu utilization into account and it should
therefore live in a separate module.

meson.build

@@ -164,6 +164,7 @@ src_files = files(
     'src/modules/disk.cpp',
     'src/modules/idle_inhibitor.cpp',
     'src/modules/image.cpp',
+    'src/modules/load.cpp',
     'src/modules/temperature.cpp',
     'src/modules/user.cpp',
     'src/main.cpp',