Add support for project hierarchy

[todoleza]

Todoist allows project nesting beside task nesting in hierarchies. It would be nice if cliist could list contents of a project and optionally it's child projects' tasks.
I noticed no visual hierarchy of projects if -A/--all argument is specified.

Anyway, great tool!

[ddksr]

@todoleza great suggestion, I will include it in one of the next releases