Offset config

[JohnMealy23]

I've added a configuration object to your module, allowing for offset borders to the viewport.

One potential use case - an engineer may have a sticky top nav that overlaps the element in question. The height of that topnav could be entered as such:

const topNavHeight = document.getElementById('topNav').style.height;
const percentViewable = vertical(el, { offsetHeight: topNavHeight }).value;

I've taken care to add supporting unit tests, and to take into account offsets when calculating percent in-view.

[coveralls]

Changes Unknown when pulling aaf887e on JohnMealy23:offsetConfig into ** on kahwee:master**.