[Feature] DumpCommand --locale-only option

Hi.
**Important**: no testing. If you think it is a good feature to implement I can make it.

I create a new option in the DumpCommand "--locale-only", which only generates the I18n routes associated to the locale passed in "--locale", and the the not I18n routes.

This can be a scenario:

default_locale: 'en'
locales: ['en', 'es']
routes:
1 - acme/no-translation'
2 - en => 'acme/foo' | es => 'es/acme/foo'
3 - en => 'acme/bar' | es => 'es/acme/bar'

```
php bin/console fos:js-routing:dump --locale=en --locale-only
```
Will dump: acme/no-translation', 'acme/foo', 'acme/bar'

```
php bin/console fos:js-routing:dump --locale=es --locale-only
```
Will dump: acme/no-translation', 'es/acme/foo', 'es/acme/bar'
this looks wrong to me. ``localeOnly`` was a boolean, and you are now replacing it with a string containing an additional extension