The default value for this limit is -1 now. Any non-negative value
will be used for the LimitFilter.

Strict recurrence means that the actual due date of the todo item is
used, instead of using today's date.

Commands shouldn't care about the format of the todo number, TodoList
will take care of that now.

Added a bunch of tests for the commands to make sure they can deal with
regexp values.

Works for delete and do subcommands.

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.