Add support to source other files in configuration

[Massimo-B]

Please add support to source other files in the btrbk configuration file in the syntax like source /etc/btrbk/shared.conf

Background is, I'm going to merge btrbk configurations of several machines. The main configuration is almost identical and could be moved into a shared identical file. Additionally it would be interesting to have support for defining variables (#493) so that the machine specific file would be only like this:

machine = "server_foo"
source  /etc/btrbk/shared.conf

[calestyo]

This, together with your #493 is similar to my #500.