Add rubber band effect.

[llinardos]

Fixes https://github.com/gordontucker/FittedSheets/issues/154 Add option to enable "rubber band", disabled by default to avoid retro-compatibility issues. Add rubber band effect (based on https://medium.com/thoughts-on-thoughts/recreating-apple-s-rubber-band-effect-in-swift-dbf981b40f35) Add demo.

https://user-images.githubusercontent.com/4202511/128634657-eadd53bd-8f81-4f6b-b99f-84222530ea30.mp4

[gordontucker]

This looks like a good addition. I'll take a look as soon as I can get some free time to do so

[iosdeveloper]

Can we get this merged please? @gordontucker