[POC] [OptionsResolver] Support nested options

Issue https://github.com/symfony/symfony/issues/4833 (which can't be fixed before 3.0) is causing unexpected behaviour in this PR. Maybe this PR has to be merged for 3.0.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | todo

Thanks @sstok, for giving this nice idea :)
So I should tag it with `@internal`?