Let import feed when required fields aren't present at data source

THE PROBLEM
Some times a data source(a csv or another) doesn't have all fields that the destination (a node or another) require. But for some business logic reason is needed to anyway import to let after it complete this fields values.

PROPOSAL SOLUTION
- Let configure the feed with a new check option "Let import when required fields aren't present"
- So when the feed will be processed, the processor EntityProcessorBase.php will let that the importer runs and create the Entities with this required fields as empty.