Add a new method to use a filter before extracting the metadata

Hi

I have notice that, when you use the doctrine:mapping:convert and doctrine:mapping:import command in symfony, the filter is added after all the metadata are extracting from the database.
In this case, when you have a table with no primary key for example, you cannot extract the mapping for only one entity.

I have also made a pull-request into the doctrineBundle and doctrine2 to fix this issue.
https://github.com/doctrine/DoctrineBundle/pull/161
https://github.com/doctrine/doctrine2/pull/603

What are your thought about my issue?