The average importance of a todo item the average of the importance of
its parents (including its own importance).

This requires some data to ship together with each todo item, since the
sorter function needs access to the parents. Because its unaware of
TodoList which maintains the hierarchy, copy a bit of that information
into the Todo itself.

* Allow relative dates for start and due tags.
* Put priority to the start when it appears mid-sentence.
* Automatically sets the creation date.

Returns None when no todo item was added.

For example:

Foo id:1
Bar p:1
Baz p:1

Would only store the ID for the last added edge (Foo,Baz) and forget
about the ID of 1 for (Foo,Bar).

Also make sure that add_edge does not add an edge twice, otherwise the
edge ID could be accidentally overwritten.

The function takes care that all edges are present in the dependency
graph.

This fixes a bug that when adding a todo with a parent which appears
later in the file, the edge wouldn't be drawn.