Adds the ability to configure the props of keep-alive on nuxt-child.
## Types of changes
- [ ] Bug fix (a non-breaking change which fixes an issue)
- [x] New feature (a non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
## Description
In development, I found it impossible to configure the props of keep-alive when using nuxt-child, like ```exclude```. This PR adds a special prop to nuxt-child named keepAliveProps to allow developer to configure it.
```html
<template>
<div>
<nuxt-child keep-alive :keep-alive-props="{ exclude: ['modal'] }">
</div>
</template>
```
## Checklist:
- [x] My change requires a change to the documentation.
- [x] I have updated the documentation accordingly. (PR: https://github.com/nuxt/docs/pull/836)
- [ ] I have added tests to cover my changes (if not applicable, please state why)
- [x] All new and existing tests are passing.
