Handle every identifier type in a separate function and try to obtain a
result from one of them.

Conflicts:
	test/DoCommandTest.py
	test/SortCommandTest.py
	test/TodoListTest.py
	topydo/lib/TodoList.py
	topydo/lib/TodoListBase.py

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.