Allow to load fixtures that have no group

[magnetik]

Hi,

When starting to use the groups, I needed to load all fixtures BUT not the one I've put in a group (that are specific fixtures that are used for some tests).

For instance when using the serialization groups in JMS Serializer, all objects that have no groups are put in a group Default.
See : https://jmsyst.com/libs/serializer/master/cookbook/exclusion_strategies#overriding-groups-of-deeper-branches-of-the-graph

It would work like this:

• doctrine:fixtures:load --group=Default: load all fixtures that have no group
• doctrine:fixtures:load --group=Default --group=User: load all fixtures that have no group and the User group
• doctrine:fixtures:load --group=User: load only the User group

WDYT ?