One item can have multiple media:content elements which may be located
in multiple media:group element to indicate that content is the same but
for the format.
This patch adds the ability to add multiple content elements and define
the group to which they go belong.
If no group is specified, all elements are located in a `default` group.
If the group is set to None, the content element is directly attached to
the item element.
Part of #58
This patch makes the feedgen flake8 compatible, fixing some minor issues
along the way. Most noticeable, this switches from tabs to spaces.
Signed-off-by: Lars Kiesow <lkiesow@uos.de>
