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
Because of the offsite info and the non-publicly-accessible links. Haven't thought through a way to mix internal and external content in the _posts yet, so figured I'd punt for now and just start with a generic "under construction"-type page. But that is a problem I'd like to tackle sometime soon after the new year.