Offer way to reverse just the last batch of migrations

[coenjacobs]

Using the down($pluginKey) method on the Handler, a plugin developer is able to reverse all their migrations. There needs to be a way to reverse just the latest batch of migrations for that specific plugin. Probably the best way to do this, is add a $batch parameter to the down($pluginKey) method of the Handler, so that just the last batch is going to be reversed.