Improve useEffect by adding dependency array + remove unnecessary duration state

[julianbenegas]

An empty dependency array means calculateWidth gets called on every render. That's no good because inside calculateWidth there are two getBoundingClientRect which are expensive for the browser.

I also remove the duration state and instead derive it from the containerWidth and marqueeWidth states.

On a local test of a big site I'm building, I'm getting much less calculateWidth calls.

Should also solve #32.