[TW-150] Create recurrences based on end dates in addition to due dates

_Cory Donnelly on 2010-07-28T17:24:45Z says:_

Sometimes the only concern a user has about a scheduled recurring task is that it shouldn't be sooner than x number of days since the last occurrence was completed. Consider the current recurrence scheme, based on due date:

$ task add due:2010-09-01 Call mom

Using alternate .taskrc file minimal.rc

Created task 3.

$ task recur:monthly 3

Task 3 is now a recurring task.

Using alternate .taskrc file minimal.rc

Modified 1 task.

Now, if the user were to call his or her mom half a month early, then the next recurrence wouldn't be for another 1.5 months.