Add fall-back support to the configuration
If a config has several files, we need to check all of them before we
can say that a variable doesn't exist.
Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Add a test for overriding config
The repo's configuration should take precedence over the global one.
Signed-off-by: Carlos Martín Nieto <cmn@elego.de>