Better support for recurring tasks

[lolli78]

Hi,

it would be very nice to have support for some kind of odd recurrence intervals like "every 3 months" or yearly (which is "every 12 months"). And it would be nice to let the user choose, if the new due date should be based on the old due date (a rather calendar-based approach) or on "now" (when the user marks a task as done), because recurring tasks are different (and I need both types).

This would be a great extension to an already great app!