Feature Request: Migrate specific bits instead of doing full migration

[bielu]

HI Kevin!
I think it would be great to be able to choose which bits we want migrate, as if we work on last fixes we want just fix some bits in Media or Content, we don't want do processing to all as it add time on top waiting to specific part being migrated.
I am happy to help with creating it if you point me where I can start looking to add it :)