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)

any particular reason why we've got `"xyz"` for the `value` but keeping an empty string for `show`?