Add snapshot of current tasks to export
Factor out filtering into single function
Remove set methods in favor of tasks.filter()

Used a monster regex:

    %s/tid \(not \)\=in tasks.\(\a\+\)_tids()/\1tasks.is_\2(tid)

Also renamed "infocus" to simply "active".
Use different internal update method
Allow title as positional argument to create()
Allow testing on multiple Python versions