Link to repo from npmjs.com

Hey there! I was looking into this project and noticed there wasn't a link from [npmjs.com](https://www.npmjs.com/package/preval.macro) to the Github repo.

Would it be beneficial to add something like this to the `package.json`?

``` JSON
"repository": {
"type" : "git",
"url" : "https://github.com/kentcdodds/preval.macro"
}
```

I'm sure there's a more sophisticated way, this was just recommended in the [npm docs](https://docs.npmjs.com/files/package.json#repository).

Would be glad to submit a PR to that effect if there's interest.

Start by inspecting package.json and compare its metadata with the npm repository field guidance linked in the issue. Add the repository metadata so the npm package links to the GitHub repository, then verify the package page displays that link.

Written by the indexing model from the issue text.