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.

In fact, the id: and p: tags in a todo are edges, so this value has to
be stored somewhere.

Also, adapt run.sh such that every test is automatically run when no
specific test was given as a parameter.

No tests though, not sure how to properly test this. Monkey patching the
datetime module doesn't appear to be working on Linux.