Recurring todos deserved some attention, since the new instance
after completing them pops up as a newly active task. This had to
be surpressed to measure the length of the todolist, and only
measure the new tasks within that range. Recurring tasks would be
added beyond that cut-off point.

The following configuration files are read:

/etc/topydo.conf
$HOME/.topydo
.topydo
topydo.conf

Values in a file lower in this list prevail over values defined earlier.

No need to set this default by the Sorter. A dependency less on
Config.

More or less works, need to make Main.py more accessible.

Still needs some mechanism to easily compose filter expressions
defined by the user.

This ANDs all arguments, resulting in the todos that match all
substrings.

After performing this command, the CLI will archive the completed
todos. This may cause the numbers to shift, invalidating the todo
numbers that DoCommand just printed before.

Make sure not to make the importance lower than the todos own
importance due to the its parents.

Test case and code were both wrong.

A todo becomes activated when all its children have been completed and
the other active conditions apply as well (e.g. start date has passed).