command: deprecate --join

As discussed in #3472.

The deprecation warning should convey three points:

  - the deprecation of `--join`;
  - the reason for the deprecation; and
  - a straightforward migration.

The reason for the deprecation is that having different files share scope is surprising. Initially the message said so explicitly, but the current wording is much shorter and still suggests that the practice is distasteful.

To simulate `--join`, first concatenate the files, then pipe the output to `coffee`. To avoid clouding this message, I think it best to ignore the possible complications: files without a trailing newline, and completely indented files. (`cat` is really just a placeholder for *an external tool which concatenates files*.)