Subfields for Migrate

After the #1676646: make migrate field handler behave the same way as term reference: accept entity labels, allow creation of entities patch will be committed, there'll be available three new sub-fields for Entityreference field:

• source_type: If the option is set to 'name', there would be used text label of an entity, rather than its ID in the source column.
• create_entity: If the option is set to TRUE (or any other value different from FALSE/0), and the source_type is set to 'name', then if no entity was found for the given label, it will be created automatically.
• ignore_case: If this option is TRUE, then case differences (uppercase vs. lowercase) between source data and existing entity names will be ignored. The default behavior is to not ignore case.

$this->addFieldMapping('field_my_entityreference', 'entityreference_source_name');
$this->addFieldMapping('field_my_entityreference:source_type')
        ->defaultValue('name');
$this->addFieldMapping('field_my_entityreference:create_entity')
        ->defaultValue(TRUE);
$this->addFieldMapping('field_my_entityreference:ignore_case')
        ->defaultValue(TRUE);