Adds timeout option to requestIdleCallback

[guanzo]

This is a modification of a previous pull request: #1513

Downloads will sometimes stall for several seconds, then resume after I click on the page, or blur the current window, or in any way cause requestIdleCallback to trigger. In some cases, the callback will never trigger if I reload the page and don't interact with it.

Delayed downloads are fine for streaming media since the user can consume the beginning chunks while later chunks get throttled, but for static media like images which need to render ASAP, the delayed download is counter productive.

I propose adding the timeout option which will guarantee that the callback runs in a reasonable time, while still allowing some idle time.

Note that the MDN docs recommend a timeout.

A timeout option is strongly recommended for required work, as otherwise it's possible multiple seconds will elapse before the callback is fired.

[Fenny]

Review please

[feross]

This seems important. Thanks!

[feross]

0.107.11