Add 7z container support

relevant Casks must use `depends_on_formula unar`
CLI - Fetch

Add `fetch` command to cli like suggestion at the end of #2528
Merge pull request #2970 from voanhduy1512/fetch_command

CLI - Fetch
Merge pull request #2744 from rolandwalker/sevenzip_containers

Add 7z container support
ability to specify container type, not detect

Fixes #2997.  Interface is optional DSL stanza `container_type`.
This should only be used in rare instances.  It is needed for the
case of a naked executable which should not be unpacked.