swaks should have a non-interactive mode that prevents any prompts

`swaks` will automatically prompt for some information when it is missing from the command line, e.g.:

```
server ~ # swaks
To:
```

This is fine for interactive usage, but counterproductive when `swaks` is used in non-interactive situations, e.g. when called from a [Bats](https://github.com/bats-core/bats-core) test suite. I'm currently writing such a test suite, and whenever I accidentally call `swaks` with invalid arguments (in this case an empty `--to` argument because the subshell expression that was supposed to generate the address failed), it causes the whole test suite to hang, without any clear indication what's actually wrong:

```
server ~/smtp-tests (master) # ./some-test.bats -f 'some'
some-test.bats
some test description 1/1
[bats hangs here because swaks is waiting for input...]
```

[As Bats suppresses STDOUT on normal runs](https://bats-core.readthedocs.io/en/stable/writing-tests.html#printing-to-the-terminal), the `swaks` prompt text isn't actually visible by default, which makes this confusing to debug.

It would therefore be useful if `swaks` would be able to detect the non-interactive usage (e.g. with [`IO::Interactive`](https://metacpan.org/pod/IO::Interactive)), and/or support an option like `--non-interactive`. When in non-interactive mode, it should exit with an error message on STDERR and an exit code > 0 when any necessary information is missing.

No source files or project tests are named. Start by tracing swaks's command-line option parsing and prompt handling, then reproduce the empty --to case from a Bats test. Done means non-interactive use never prompts, reports missing information on STDERR, and exits nonzero.

Written by the indexing model from the issue text.