Move old @done to filename.todo-done #enhancement

[hahiserw]

I think it would be nice to have an archive like this.
For example if your todo list is very long and it is full of @done tasks.
It would be more clear

[aziz]

The ideology behind PlainTasks is to keep everything in a single plain text file.
If your archive list is getting long it should be fairly easy to copy and paste them to another file for archiving.
It could also be written as a separate script (in any scripting language) and I'll be happy to include or link to in this repo.
All in all, it should be separate from the plugin.