Add support for requesting multiple devices

[reillyeon]

The use case here is that if there is a process which may be repeated over a set of devices or a set of devices act in concert (maybe a bunch of individual WebLights being used for different indicators) then it is a poor user experience to manually repeat the process of selecting them one at a time. The simplest proposal would be a navigator.usb.requestDevices() method which returns a Promise<sequence<USBDevice>>.