Preload JS and CSS assets also

[gigabo]

#74 introduced data preload to improve client transition performance between pages on the same route, but fetching static assets will still drag down client transition performance across routes.

@aickin proposed a solution that supports not only prefetch of webpack asset bundles, but also any independently included assets required by a page.

This would significantly improve first-time client transition performance across routes.

[aickin]

As @bartkusa noted, there may still be a bug in Chrome that makes rel=prefetch links have higher priority than they should.