use `json` query param value to traverse context objects

In development (and in product for npm employees), it's possible to examine the view context of any page by adding a `json` param to the query with no value:

[/~shtylman?json](https://preview.npmjs.com/~shtylman?json)

This patch allows the value of `json` to represent a deep key in the object:

[/~shtylman?json=profile.meta](https://preview.npmjs.com/~shtylman?json=profile.meta)

I don't think `show` is used any more. The user presenter now only uses name and value to create the `meta` object: https://github.com/npm/newww/blob/9078eefc6bd461f3980b91680cc5abe7c5f8a631/presenters/user.js

This user/profile presenter stuff definitely needs to be refactored and tested at some point, but it's not directly related to this PR.