Before `Setting.a.b.c` had `@section == "'b' section in 'a' section in
'[PATH]/settings.yml'"`, now it has `@sections == ['b', 'a',
'Settting']`. The purpose of this commit, is to have the section's
"stack" available and not just a string.
The only use of `@section` was to show a meaningful MissingSetting
message, this is not change in this commit and the message is so
similar, all tests are passing.
If set to true, non existing settings attempt to retrieve value from
environment variables. For example:
`Settings.non_existent_key == 'foo' ` if
`ENV['SETTINGS_NON_EXISTENT_KEY']='foo'` and Settings has