Adds `package-lock.json` to `.gitignore`

[benjaminhoffman]

Going through the service worker course on udacity and running npm install generates a package.json file thereby automatically creating a diff. May confuse newb users when they run npm reset --hard as there is still a diff.

[benjaminhoffman]

@jakearchibald would love to get this merged. Helpful for engineers who are just getting started w/ offline-first courses. :)

[benjaminhoffman]

@jakearchibald bump :)

normally this isn't that big of a deal but it does create confusion for newb devs who may not understand why there's a diff.