Add an option for class names

[iamvdo]

Defaults class names are .past, .future, .inside and .seen. It would be better to add an option for that names.

[vdcrea]

Hi Vincent and thanks for this plugin I am testing now.

I am not sure if I use it well but I don't see .future class. only .past .seen and .inside are applied. Also, .inside class is applied only after .seen class. It means that elements immediately visibles don't have .inside class.