Add some new lifecycle hooks for components

Hello,

I would like to propose add some some new lifecycle hooks for components including async ones.

Now it too difficult to know when async components are loaded and you need to add some wrappers:

https://github.com/vuejs/vue/blob/dev/test/unit/features/component/component-async.spec.js#L62 https://github.com/vuejs/vue/blob/dev/test/unit/features/component/component-async.spec.js#L70

It would nice to use built in lifecycle hooks instead:

import Vue from 'vue';

const vm = new Vue({
    components: {
        component1: resolve => {
            require(['./component1'], resolve);
        },
        component2: resolve => {
            require(['./component2'], resolve);
        },
        component3: resolve => {
            require(['./component3'], resolve);
        }
        // etc
    },
    ready() {
        console.log('All components are loaded', components);
        // For instance, you may use `expect` here for tests
    }
});

vm.$mount();

[mdreizin]

@posva When you have one async component it will be easy to handle it, but when you have more than one async components it will be difficult 😢.

[mdreizin]

@yyx990803 @posva Thanks guys! Currently I have found a workaround and if I create really common solution I will post here.