WIP: multiple URLs per Cask

- Work-in-progress because there are no docs/tests at present.
- Permits `url` stanza to be repeated multiple times
- checksum keys such as `:sha256` may be appended to each `url`
- `:target` may be appended to `url` to specify a target download name,
  with special value `:basename` indicating the basename extracted from the URL
- `:multipart => true` may be appended to `url` to indicate assets which will
  not be subjected to container analysis or explicit unarchiving.  `:multipart`
  may not be the best name for that. `:fetch_only` ?