[Form][Translator] Added `choice_label_format` option

| Q             | A
| ------------- | ---
| Bug fix?           | no
| New feature?   | yes
| BC breaks?     | no
| Deprecations? | no
| Tests pass?   | unsure (see https://github.com/symfony/symfony/pull/12882#issuecomment-66195601)
| Fixed tickets | #12880
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/4602

### What this adds...
Inspired by the recently added `label_format` option, I wondered if this was worth merging.
Basically, these changes allow us to do this:
```php
<?php
$builder->add('gender', 'choice', array(
  // ...
  'choice_label_format' => 'my_form.choices.%name%.%choice%'
));
```

My reason for adding it to the `BaseType` (just like the `label_format` option) is that you could then also define this option on the form itself during construction. It might seem odd to do this but consider the convenience of doing this (within a framework project):
```php
<?php
$searchForm = $this->createForm(new SearchType(), null, [
  'label_format' => 'docs_search.form.fields.%name%',
  'choice_label_format' => 'docs_search.form.choices.%name%.%choice%'
]);
```