Adds support for (un)hiding "future tasks"

When parsing tasks, look for the `t` (aka threshold) tag. If the
threshold date is in the future, then the task is a *future task*. A new
preference controls the global (un)hiding of such tasks.

This adds support for todo.txt's [futureTasks](https://github.com/ginatrapani/todo.txt-cli/wiki/Todo.sh-Add-on-Directory#wiki-futuretasks) filter, which is part of the official add-ons.