validate input on blur

[notwaldorf]

This matches the behaviour in these other PRs:

• [ ] https://github.com/PolymerElements/paper-input/pull/148
• [ ] https://github.com/PolymerElements/gold-email-input/pull/29
• [ ] https://github.com/PolymerElements/gold-phone-input/pull/28
• [ ] https://github.com/PolymerElements/gold-cc-cvc-input/pull/23

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project, in which case you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[cdata]

LGTM :rocket: Looks like conflicts must be resolved, though.