Disable scrolling on mobile devices [#273]

[jameswilson]

PR followup to #273 to add proper cross-browser and device-compatible approach to restricting viewport / page background scroll while a featherlight is open.

[jameswilson]

This solution is how Facebook does it, and is loosely based on this answer to this post on stackoverflow.

https://stackoverflow.com/a/45230674/413538

[RavanH]

+1