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.

add_dependency would store them as strings. On the next invocation
of add_dependency, find_new_id wouldn't find the previously stored
value since it looks for integers. Therefore it returns the same
ID, resulting in duplicate edge IDs.

There's a difference between 'text' and 'source text', where the former
is a more user-friendly representation of a todo, while the source text
is the raw text from the todo.txt file.

Reflect this difference in the method name to avoid confusion with the
'text' method.