Task bullet space added.

`tasks_bullet_space` allows setting the bullet and margin space type.
This makes it possible to fully use of tabs instead of spaces. For
instance, The following configuration will work nicely in PlainTasks:
	"tasks_bullet_space": "\t",
	"translate_tabs_to_spaces": false,
Having a Tab as a separator allow the done, cancelled and pending tasks
to always align without "jumping" when state changes.