Added package.json for npm usage

[hexpunk]

I, as well as many others, use npm for front-end package management. For example, using with webpack. This would add support for npm.

I would recommend you add this to the npm registry, but that's not strictly necessary.

[googlebot]

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[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.

[hexpunk]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[samthor]

Thanks!

[hexpunk]

No, thank you!