Add in node / npm versioning.

[pwhisenhunt]

Since we're getting peeps hacking on this we should specify the node / npm version used in the project; e.g. I wanna add a new npm package but I'm not sure which node version we're using. Apologies if this is mentioned somewhere.

Ideally, we should probably add:

• npm and node to the engines section of the package.json file https://docs.npmjs.com/files/package.json#engines
• I'd love a .nvmrc file with the node version so I can just nvm use when hacking on this project

@stephanieinez what node / npm version did you use to develop the project? We can probably just use that.

[grady-lad]

@pwhisenhunt The versions I have been using were:

• Node: v9.2.1

• Npm v5.6.0

We could probably go ahead using them versions?