Support for container parameters inside behat.yml

Hi

I'm looking for a way to use global parameter in Behat 3. I need to use same parameter in many contexts and defining it by hand will be "bad" thing.

My idea is to be able to use parameters like in Symfony 2 when I can import them from external yml file.

```yaml
parameters:
    baseurl: http://google.com

...
    Acme\Context:
        - %baseurl%
```

What's your opinion for this?
Also asking because maybe there is some way to do this with current code and I missed it reading the docs ;)