Support for passing custom component to as prop of Link component.

[pascalbaljet]

Fixes #1746. Supports custom components instead of just HTML tags:

<Link as={CustomButton} method="post" href="/user" data={{ test: 'data' }}>
    Custom Component with Data
</Link>

Replaces PR #2264 and #1940 (will add you as a co-author). I've opened a new PR as this builds upon PR #2441 and I don't think Inertia should alter DOM elements. It now always passes the href attribute to custom components and it's up to the user to handle this (or not).