Also modify the keys for the dependency graph, use the hashed version of
the todo instead of its position.
The position may shift when deleting tasks, which affects the consistency
of the dependency graph.

Now, the todolist is the owner of the numbers.

A step towards elimating knowledge of the todo list in the items itself.

A step to get rid of todo.attributes["number"].

are passed to that subcommand.

This is achieved by overriding the start position of where to take the
arguments in argv.

Also make a pretty printer for a single todo.

Instead of numerous booleans that steer the pretty printer, let the
caller pass on the desired transformation function.

This will mark the todo list dirty.