Set default value to Config.IGNORE_WEEKENDS.

No need to set this default by the Sorter. A dependency less on
Config.
When -x is given, do not add the LimitFilter.
Implement 'after', 'before' and 'partof' operators.
Introduce configuration files.

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.