Load override feature toggles if present

- Extract a class for loading feature toggles.
- Load an override feature toggles file if present.

This allows us to override feature toggles on a per-environment basis.

@pbadenski
@robyoung
Refactor feature toggles to be more Rails like

It seems as though application logic should not live in the config directory so FeatureFlags has moved to lib.
Keep yaml configuraton together in config dir
Merge pull request #96 from alphagov/environment-specific-feature-toggles

Environment specific feature toggles