Scheduling: allow to schedule tasks for the last day of month

There's a small feature I'd like to file to not forget about it. Probably it could be implemented in some bigger scope, when designing the scheduling system, but for now let it to just be here.

Taskwarrior periodic/recurrent tasks lack flexibility, and one particular feature I need for my schedule is to be able to schedule tasks to be performed in the last day of month (e.g. some rent payments or whatever).

When we'll be designing our scheduling system, we'll need to add this feature beyond the "basic" Taskwarrior-like feature set.

This task depends on the following:
- [ ] #6 (since task schedule will likely be an attribute)

The dependency list isn't closed, since the scheduling feature isn't planned yet.