Allow get parameters by path, separated by colon

For example:
#app/config/parameters.yml
parameters:
    social:
        facebook:
            app_id: 1234567890

Now it's possible to get app_id like:
$this->get('social:facebook:app_id');

The colon symbol ":" don't allowed for use in parameter name and it's the most appropriate symbol for separator. 
At last we get more conveniently for get nested arrays value.