Adds support for 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.
Fixes #30 : Merge branch 'feature/hide-future-tasks' of
https://github.com/chaghi/QTodoTxt

I just add a Filter in order to stay consistent with others part of
QTodo