Add 'startAtBottom' flag

[dbkr]

Controls whether a scrollPanel starts off at the bottom.

This may not be necessary and could either be derived from stickyBottom, but
this means I can be sure that the behaviour of ScrollPanel is completely
unchanged for all other uses to avoid breaking any other uses of
ScrollPanel.

[richvdh]

I think you probably just wanted this.scrollState = {stuckAtBottom: this.props.stickyBottom}. I understand your conservatism, however. On the other hand I'd prefer it if the comments saying that you think it's just conservatism made it into the code rather than being lost in the PR.

[dbkr]

Good point: documented on the param

[ara4n]

lgtm