Add dates to the written files

[daniellandau]

The completion date is mandatory as per the format and the creation date is also
useful. These are not shown to the user yet in this todo.txt client, but are
used in many others, so it's useful to include them already in the file even
though we don't yet have an implementation for showing them in the UI.

Your implementation of the parsing is much cleaner. Showing the dates (and sorting by them) would be good, but for the time being I'd be somewhat happy even with just saving them as I also use the todo file on my phone and there I already sort by creation date. I can't really use Go-For-It at all if it doesn't include creation dates in the file as then it messes up the file for the other client.

[JMoerman]

The completion date is mandatory as per the format and the creation date is also
useful.

According to https://github.com/todotxt/todo.txt neither completion date nor creation date is mandatory. However, if a completion date is present a creation date must also be present. This means that with the changes from 9c877b0 Go For It! will generate invalid todo.txt lines for tasks which did not contain a creation date.

[daniellandau]

This is the section I'm reading to get my conclusions:

If you’ve prepended the creation date to your task, on completion it will appear directly after the completion date.
[...]
With the completed date (required), if you've used the prepended date (optional)

[JMoerman]

Allright, that means that a completion date is mandatory if and only if a creation date is present in a completed task.

[daniellandau]

What, no it doesn't. Without the comma it might, but as it stands no way. The if sentence there is related to

if you've used the prepended date (optional), you can calculate how many days it took to complete a task.

[JMoerman]

That last sentence is a bit weird, but from (a):

If you’ve prepended the creation date to your task, on completion it will appear directly after the completion date.

and (b):

Rule 2: A task's creation date may optionally appear directly after priority and a space.

and, finally (c): https://github.com/todotxt/todo.txt/blob/master/description.png

we get the following rules:

1. A creation date is optional (b, c)
2. If a creation date is present a completion date must be present for completed tasks (a)
3. If a completion date is present a creation date must be present (c)
4. A completion date is optional (c). (Also follows from 1. and 3. combined)

[JMoerman]

I was waiting on an official response on todotxt/todo.txt#21, but that doesn't seem to be happening any time soon.
I merged it, but also added a check for the existence of the creation date ensuring that either both exist or neither exist for finished tasks.

[daniellandau]

Sounds good, thanks!