add onBeforeActivate and onBeforeDeactivate hook

[danyadev]

What problem does this feature solve?

1. onActivated and onDeactivated are the only hooks that have no option with the Before prefix.

2. I need onBeforeDeactivate to maintain the scroll position when closing or switching between components. In a little more detail I described this in another issue.

What does the proposed API look like?

onBeforeActivate(() => console.log('call #1'));
onActivated(() => console.log('call #2'));
onBeforeDeactivate(() => console.log('call #3'));
onDeactivated(() => console.log('call #4'));

[Hisioni]

why not use vue-router hooks

[eolme]

why not use vue-router hooks

Why do I need vue-router to switch 2 components?

[HcySunYang]

Closed, tracking here vuejs/rfcs#284