[Ml-yokadi] Using '_' as a shortcut to last task id

[Aurélien Gâteau]

Hi,

Please find attached patches implementing '_' as a shortcut to last task
id. This means you can do:

  yokadi> t_add home Buy chocolate
  Added task 'Buy chocolate' (id=1069)

  yokadi> t_edit _

To edit last task.

It should work in all task-related commands (please report if you find a
missing command). I choose '_' because Python interpreter use the same
symbol for the same purpose.

What do you think about this?

Aurélien