Allow `onClick` on `next/link` child. This should not be a breaking change, but it's a very useful feature. Real-life use cases include: analytics or closing menu on navigation, and other.

- [x] allow optional `onClick` on `next/link` component's child
- [x] call original `child.props.onClick(e)` before `this.linkClicked(e)`
- [x] add integration tests
- [x] cancel the navigation if `e.defaultPrevented === true`

Fixes #1490