Enable `thisweek` option, add month options for due dates

[ltrubov]

Addresses the bug described in https://github.com/ultralist/ultralist/issues/258

Enables filtering tasks due the current week, treating Monday as each week's start. This brings the functionality in line with what's described by the help pages, producing valid results instead of errors when running a command such as ultralist list due:thisweek.

In addition, passing in due:nextweek and due:lastweek filters will get the tasks for the entire week, rather than just the corresponding Monday as it is currently.

When adding tasks, the due:nextweek and due:lastweek will set the due date to the Monday of the appropriate week (preserving the current functionality), as will due:thisweek (instead of an error as it is currently).

Finally, lastmonth, thismonth, and nextmonth date options are added, with the same functionality as the week flags. When setting a task's due date, it will be set to the first day of the corresponding calendar month, and when used in filtering, will filter out all tasks with due dates in that calendar month.