fix: Forces Link to expose the href to the child using passHref property.

[nihguy]

Fixes #1942 and other situations in which to expose the next/link href attribute for tag is necessary for avoiding problems with SEO.

The solution proposed by @arunoda, in the old #2475 solves the problem, adding a boolean passHref property to Link, forcing it to pass its href (or as) attribute to the child, when the correct type of tag can't be recognized.

It could be used like that:

import Link from 'next/link'
import Unexpected_A from 'third-library'

export default ({ href, name }) => (
  <Link href={href} passHref>
    <Unexpected_A>{ name }</Unexpected_A>
  </Link>
)

[arunoda]

This is great.
I'd like to see following things:

1. A section on README under here: https://github.com/zeit/next.js#with-link
2. A simple test case somewhere here: https://github.com/zeit/next.js/blob/v3-beta/test/integration/basic/test/client-navigation.js#L35

Then let's take this.

[arunoda]

This is awesome.