Support Mulitple Enclosures

[cristoper]

The version of feedgenerator.py distributed with Django supports passing an array of 'enclosures' to add_item() instead of a single 'enclsoure'. (Multiple enclosures are only supported by Atom feeds.) Here is the PR that added the supporte for multiple enclosures (August 2015):

django/django@aac2a2d#diff-1d2005672da1027b84aaeb48fe105351

It would be nice if that could be brought over to this package.