Add settings file discovering

Introduce a new way to load settings file with a discovering
which either just load given an explicit file path or try to find
it from base directory and available settings backends. This is
related to issue #30.

This should not include backward incompatible behavior, it just adds
capacity to find another backend default filename kind.

Concretely, before this release only 'settings.json' would be finded
when no explicit file path was given and now it will be able to find
also a file 'settings.yml' if it exists in current directory.