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
Can we clean this up a bit too? something like:

```js
let instances = target.__ractive_instances__;
if ( instances && instances.length ) {
    removeOtherInstances( instances )
}
```