Add "Config" module.

Add "Config" module.

This module contains functions to read, validate and set various
configure options which are used in guardian.

The main functions called "UseConfig" allows to pass which config file
should be read and validates the input by calling a subfunction. The
validated settings will be returend as a hash.

The "CheckConfig" function directly can be called with a hash, which
contains the various config options and values and will return an
error message if there are any problems.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>