Add ability to sort tasks by reminder date

[brarcher]

I've use this application for a little while now. It really helps me organize and not forget to do various tasks. Thanks so much for creating this application!

One suggestion I may make is in how the tasks themselves are sorted. Currently they are sorted by the date they are added, where the oldest tasks are at the top. It would be useful to be able to sort them by the reminder date. If this were so, one would be able to look at the list and quickly determine which tasks apply for the given day.