Only publish v3 snippets in public mode

v1 had no public/private distinction. v2 was public-by-default. v3 is private-by-default.

#14 should be merged before this PR.

@afeld Care to review?

@OriHoffer FYI
I probably would have written this as

```ruby
text = snippet[field] || ''
```

and gotten rid of the conditional. The tradeoff for eliminating the branch in logic is that it unnecessarily runs the rest of the body of the loop, but at least it's consistent.