Add support for bulk indexing

[clintongormley]

Add Elastic::Model::Bulk to save/overwrite documents in batches for improved performance:

$bulk = $model->bulk(
    size        => 1000,
    on_conflict => sub {...},
    on_error    => sub {...}
);

$bulk->save($doc);
$bulk->overwrite($doc);
...

$bulk->commit;