[PROPOSAL] :event:then(:event, ...) selector

Hi all,

Tiny proposal about a then selector, useful to mais some transitions, etc.,
when events are chained.

Example :

```
/* triggers when the focused ol loose the focus, if the ol is still
hovered  */
ol:focus:then(:hover) {
  transition: width .3ms;
}
```

Useful, for example, to close a focused list, but after a possible click
effect, or to show a message, when an input pass from :invalid to :valid,
etc.

What do you think about it?

Michaël Rouges - https://github.com/Lcfvs - @Lcfvs