[feature] Shadow DOM support

[twilson90]

Seems a bit odd that it hasn't been implemented considering Shadow DOMs have been around for a while now.

Whenever I've tried using SortableJS in a Shadow DOM enviornment, there's a bunch of issues. Some are easy to resolve with a quick hack, but others are not.

Fairly simple to implement, mainly just replace instances of 'document' with the sortable element's root node w/ Element.getRootNode().

[SalesforceBobLightning]

Any word on when this might be completed? :)