Make creation date optional (in file and UI)

[Vrihub]

Congrats for the app! It really looks very promising to replace simpletask.

One issue I found after importing my todo.txt to test the app is that the tasks creation date (automatically set to "Today" for imported tasks that are missing it) is always displayed as a label under each task.

To me it clutters the list view with unuseful information for tasks without a due date, and it causes confusion for tasks with a due date. Most of my tasks the creation date is irrelevant, and I'd like to see only due dates in the task list.

Suggested changes:

1. When creating a new task, set a default null creation date (like we already do for due date). Currently it's impossible to add a task without a creation date.
2. Don't automatically add today's creation date to tasks that lack it when reading a todo.txt file
3. Add an option to let the user turn off the visualisation of creation dates in list view

Suggestions 1. and 2. would make the app compliant with the todo.txt specification, which defines creation date only as optional (see https://github.com/todotxt/todo.txt)

Suggestion 3. Would be a nice feature to have even when using creation dates in todo.txt file.