Consider forwarding transition and animation directives

[rgossiaux]

We already forward use: (via the use= prop) and on: (via some runtime hacks). We should consider supporting equivalents for the transition/animation directives too: something like in=, out=, transition=, and animate=. Not sure if any other popular component library supports this, but I think it's probably pretty useful.

[rgossiaux]

After sitting on this for a while, I'm not sure if this is worth the added complexity. I think it's fine to ask you to just use a wrapper <div> for the transition directives. If anyone would actually want this feature, feel free to leave a comment here. Until then this is going into the backlog.