Right now, can only set the default task for a Collection via the decorator, which is kinda dumb given one can mix & match tasks within collections. IOW the concept of default-ness is if anything more an attribute of the collection than the task, so not even being able to manage it via the Collection is pretty dumb. Conversely, a task may not want to be the default in all collections it's in, only some.
Easy enough to add a kwarg to add_task to solve all this.
DDD re: #109
Failing tests re #109
Changelog fixes #109