Move existing instance removal from init to render

If the instance isn't set to append, other instances on the target element should first be removed/torn down.

I don't foresee any issues with moving this, and all the tests are happy. I'd like to get another set of eyes and cerebral cortices on it, though.

Fixes #1430
The general policy is to have identifiers at the top of functions, which has always seemed a little odd to me. I understand why.... because, javascript. So with `let`, it's ok to move away from the convention?