Implement flags for CLI script.

-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.
Merge branch 'opts' into stable

Conflicts:
	topydo/cli/Main.py
Merge branch 'stable' of github.com:bram85/topydo into stable