The syntax pretty much the same, but its applying cape instead of skins.

implemented in 2.0.0 (via GUI)

This would be possible client side only, so it's not the highest priority. It will be implemented after #12 and #22 , if I have time and motivation.

Oh also, if we in server for example, and other people also have the client, can they see my cape? I think its possible, just like the CustomSkinLoader

No, the way it would be implemented other players wouldn't see the cape, as you'd need a cape server otherwise.

But why the CustomSkinLoader mod thing can do that? Is that different?

CustomSkinLoader uses online services afaik, so all the clients get the info about the cape. At least I think so.