Add subgroups to allow sorting within a group
Add subgroups to allow sorting within a group

Example .formatter.yml:

use-sort:
    group:
        - [Symfony\Component\HttpKernel, Symfony]
        - _main
        - App

to put Symfony\Component\HttpKernel classes on top of the group

Fixes #10

jumbojett/php-formatter,mmoreram/php-formatter