Macro implementation of the JSX plugin's v-slots directive

[craigrileyuk]

Forwarding slots from one component to the next inside the <template> is always a bit of a pain. The Vue JSX plugin has a great directive called v-slots which enables you to forward slots in two words.

Would be great to get a usable version of this available for experimental use. Great outline for how it could work here: vuejs/core#575 (comment)

Proposal

// Wrapper.vue
<SomeComponent v-slots="$slots">
    <template #foo> 
        Bar
    </template>
</SomeComponent>

will forward any slots sent to Wapper.vue to SomeComponent and overwrite any manually defined ones (such as "#foo").