New options to fos:elastica:populate command: offset, sleep, batch-size

I've added some new options to the `fos:elastica:populate` command.
I've also modified the `ProviderInterface::populate` function signature: added the `$options` array parameter as the second parameter.

The `fos:elastica:populate` command now passes the `$input->getOptions()` array to the `$provider->populate` call, thus providers can be controlled by the command parameters.

New options in `fos:elastica:populate` command:
 - `offset`: indexing offset can be given (default: 0)
 - `sleep`: sleep time (in microseconds) between object persisting iterations can be given (default: 0)
 - `batch-size`: indexing batch size can be given, it overrides the one configured in the provider