Check online status before prefetching URLs

[ghost]

Related to issue #73

As a website owner, I want to only prefetch URLs when I am online, so that I am not getting network errors when my site is offline or served by a service-worker.

Changes:

• Implement isOnline function to check online status via navigator.onLine (https://developer.mozilla.org/de/docs/Web/API/NavigatorOnLine/onLine).
• Implement guard in preload function to check online status with this function.

Only tested on my setup manually. No test suite was run. Should be tested on larger scope before merging.