Set id to something unique when new fields are spawned

[jechols]

I'm not sure if it affects users in any tangible way, but right now adding multiple fields results in them all having the same ID. This will make testing a bit more painful, and I don't really know the ramifications of violating the HTML spec. id is supposed to be unique, but beyond testing I'm not sure if there are any actual problems.

In any case, if it's easy enough I'll try to throw together a PR to fix it, and if not, it's probably worth noting at least.