Add support for variables substituion similar to I18n.

Hey!
I thought it would be nice to have variable substitution, just like `I18n` does it, since `YAML` does not allow string concatenation and **partial** existing parameter substitution.
So, assuming we have the following YAML-file:
```yaml
defaults: &defaults
  name: 'Defaults'
  mailer:
   host: 'localhost'
  email: '%{name}@%{mailer.host}'
```
`email` will be equal to `Defaults@localost`. 
Local settings and nested keys are supported too, see tests for more.