Add ability to specify "viewport" padding

[satazor]

This is a follow up of an old issue I reported, but essentially it would be cool if we could have a way to prevent the source to touch the viewport edges.

[satazor]

See: https://github.com/yiminghe/dom-align

[charlieclark]

+1 agree that this would be a really handy feature. It would be great to be able to prevent an aligned element from being clipped by the viewport.