[Enhancement] Allow continue-on-error when insert duplicate records

[tamnguyenvt]

In mongodb i created an unique index on collection. This is to make sure we dont have duplicated records on database when logstash reprocess a file. But currently we cant do this silently and logstash keeps throwing error sth like An unexpected error occurred! {:error=>#<Mongo::Error::OperationFailure: E11000 duplicate key error collection:
This can be fixed by this solution. https://stackoverflow.com/questions/34153058/mongodb-multi-document-insert-ignore-custom-duplicate-field-error

Please can this be improved because its really helpful ?