Support task decorator without arguments

[jhthompson]

Closes #48

Inspired by:

• Django's @login_required source for the general structure
• Real Python's "Creating Decorators With Optional Arguments" for the asterisk syntax

The use of @overload for typing is new to me, but comes straight from mypy's documentation on decorator factories:

Sometimes the same decorator supports both bare calls and calls with arguments. This can be achieved by combining with @overload ...

Adam's great post "Python type hints: how to use @overload" was also helpful in understanding that concept.