Add Container: Prop to docs #577

Hey there please add container: prop/option to the docs as i was struggling for so long to set this to a node instead of an id string..
i needed a node to handle multiple containerIDs that were the same..

i found container option in the code but nowhere does your docs mention this, it only says containerID.

container: document.querySelector('#list')

worked perfectly for multiple #list scroll containers

Thanks