logilab-common #5490 allow changelog to add sub message to message [open]

Changelog should be able to add sub_message to Changelog

eg:

* Message

    - Sub Message
    - Sub Message

I think we need

1. a -sub options
2. to allow this option to select to which message you want to add this sub message

eg: select Message A or Message B for

* MessageA

    - Sub Message
    - Sub Message

* MessageB

    - Sub Message
    - Sub Message

Note that changelog.py deserve a ChangelogMessage class to clean up the implementation.