Allow for an initial render or pace.op(0)

I find it looks much better to have an empty progress bar initially rather than nothing at all. I'm currently doing an initial render by doing this: `pace.op(-1); pace.op();`. I would rather just use `pace.op(0)`, but this doesn't work. The count check shouldn't test for a truthy value. Instead, it could just check `arguments.length` or `typeof count === "number"`.
