Fix enhance & append conflict check

[monoblaine]

Description:

Preface: We should not use enhance: true and append: true at the same time, so Ractive checks for this case and throws an error if we misbehave.

However, if we use Ractive.defaults.enhance = true; and create a Ractive instance with append: true, Ractive fails to throw an error.

That's because here

// disallow combination of `append` and `enhance`
if (options.append && options.enhance) {
  throw new Error('Cannot use append and enhance at the same time');
}

we're using the options object to check for the conflicting case. I guess we should use target instead of options to do this.

Is breaking:

It was breaking the Cannot use append and enhance at the same time test in enhance.js because options.enhance was not getting copied into target. And that's because the defaults object does not include enhance. That's why I added that in 4bdefbd.

[evs-chris]

👍 Thanks!

Typically I'd ask for a test as well, but this is a very simple change and tests with global state are icky 😄