Provide promting and non-interactive modes for commands when applicable

[baptistemehat]

For instance, to add a new task with a parent, provide both:

$ forest add "My new task" -p 45b666f9 -e

and

$ forest add
> Task name:   My new task
> Task description [y/n]:  y  (opens editor)
> Parent (hit enter to skip):   45b666f9