Add "sticky-bottom" option to change sticky position

[ghost]

Hi @rguanghui ,

It would be nice to have option to switch from sticky top to sticky bottom position. I think it could be option sticky-bottom or inverse mode, and if sticky-top: undefined then code starts to use sticky-bottom value.

[seyfer]

also to the author

is it possible to use the custom class name when it is sticky?
and please add the demo page to the github pages.