DSL: add `artifact` stanza

Keeping the `link` stanza in the DSL is a mistake: it will be
confusing when combined with the ability to install by copying.

This PR retires `link` completely, and adds a generic artifact
stanza called `artifact`.  (`link` is removed from the docs, but
will still work for compatibility during transition.)

This only affects one current Cask (dwarf-fortress.rb), and that
Cask may be changed to use `suite` in the future.