Delay progress bar if page is loaded within N ms

[switz]

https://github.com/rstacruz/nprogress/issues/169

I don't want to show the progress bar if the page loads in under n milliseconds, seems unnecessary for pages that load very very fast (which is most of the time).

[drdla]

Anything below ~ 500 ms does not need an indicator from a user's perspective. Always showing the progress indicator in fact deteriorates user experience for fast page loads, because it is a rather distracting flash.