New stamp.composeWith(stamps) function.

I decided not to create `.composeAfter()` because IMO it most likely won't be used by anyone.

This PR increases `stampit.min.js` file size by 70 bytes. From 6883 to 6953.
This will slow down compose because it has to jump through all the hoops to initialize a new stamp every time it runs. That's not necessary. `.composeWith()` could have called `.compose()` and passed `this` as the first parameter, right?