Preload all Route assets

[lukeed]

Taken from: https://github.com/sveltejs/sapper/blob/master/lib/index.js#L148

At the very least, styles should be preloaded via the server. I'd argue that fonts & co should be set as a <link rel=preload/> by the user from within the template... but the server should send down the bare minimum assets.

[antony]

@lukeed the original code for this is gone, so it's difficult to understand the issue.

Was the goal that things like styles and client js should have a rel="preload" attribute? Somebody was suggesting this the other day in chat.

[lukeed]

Yup. Fail on my part for linking to master

[benmccann]

This sounds like a duplicate of #408

I actually have a PR pending #1269 that will preload the JS and CSS, which has been pretty impactful in my testing