New scroll align mode: 'smart'

[gaearon]

This adds implementation + tests. I can write the docs if you'd like them to be a part of the PR.

The purpose is to add a new scrolling mode called smart. We may make it default in v2 if we want.

It behaves like auto when the target is within a single screen from the viewport boundaries, and behaves like center otherwise. The rationale is that if something is far enough, there's no meaningful context to preserve — and therefore we might as well center it.

In particular, centering is helpful for cases when the next selected item is relatively random (such as with "find by DOM node" picker in DevTools). In that case, the auto behavior often jumps between the top and the bottom selected nodes because we're not necessarily scrolling in any particular direction. However, the smart behavior tries to center things unless we're already pretty close to them.

Before

After

[bvaughn]

Thanks Dan! I updated the docs site.

Published as v1.8.0