Add support to source other files in configuration

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
```

Start by locating btrbk's configuration parser and the handling of the `/etc/btrbk/shared.conf` configuration file. Trace how a `source` directive would be read, then verify that a machine-specific configuration can load the shared configuration and that parsing completes successfully.

Written by the indexing model from the issue text.