dns-prefetch option

A nice feature request based on the latest article from piwik:
https://piwik.org/blog/2017/04/important-performance-optimizations-load-piwik-javascript-tracker-faster/

<link rel=”dns-prefetch” href=”//example.innocraft.cloud”>

could be added to the header on every page with the domain of the remote piwik installation. Should give you 10-50ms speedup for first time visitors.