`timer_spin()` definition needed

[AshlinHarris]

The function is not defined for empty input, so @spinner :clock x=2 works but @spinner x=2 fails.

[AshlinHarris]

As a quick fix, #65 adds another definition of timer_spin, but this approach won't be feasible if the number of arguments to timer_spin() increases. It would be better if timer_spin or _check_inputs set default values.