If that configuration file lacks options, the defaults will be used.

This will be used to make sure that tests always run with a fixed,
well-defined configuration, not influenced by how the user happened to
configure topydo.

When an expression 'due:' is used, it should be treated as a normal
filter and not as an OrdinalTagFilter.

Conflicts:
	topydo/cli/Main.py

-c, -d and -t are supported to specify alternative configuration files,
archive files or todo.txt files respectively.

The configuration file specified with -c has the highest priority.

The files specified with -d or -t are always used regardless of the
configuration.

No dependency tracking is needed and gives a huge performance
improvement for huge done.txt files.

The subclass implements the dependency tracking of todos, which can be
quite intensive for huge todo.txt files, and is not always needed. For
instance, dependency tracking is not needed for archiving.