[WIP] allow user to override amp-img placeholder and hook into active and hidden class

[erwinmombay]

No description provided.

[cramforce]

Why don't we allow overriding the default image placeholder classes?

[erwinmombay]

@cramforce i think that was just because we wanted to start inclusively with the placeholder and start opening up the hooks. after playing around with it some more, i don't think theres going to be much harm with making -amp-autoplaceholder (private) into amp-autoplaceholder (public) as the developer can shoot themselves in the foot anyways if they add their own placeholder so might as well make the default classes public.

[erwinmombay]

/cc @dvoytenko if you have any concerns on making the default auto-placeholder classes user stylable.

[erwinmombay]

for reference => #339

[dvoytenko]

I could see making them immediately public if we didn't have special HTML in there to make it work, e.g. loader dots. I'm not sure we are quite ready. We have to make it customizable, but I'd prefer to do it post preview.

[cramforce]

Fine with me!

[dvoytenko]

Looks like we can close it? I'm closing. Please reopen if disagree.