Add archive command

In #373 I change `make cache` to also archive old bootcamps. @gvwilson notice that 

> if someone's working on a blog post, and they just happen to use to be the first to cross a clock boundary, they shouldn't be distracted by the update of bootcamp URL files.

This PR restore the previous `make cache` and create a new command `make archive` for the change version of it.