Expand environment variables in config files

I'm not sure if this is a good idea, but someone tried it and found it didn't work. Noting it here for future consideration. Perhaps if it is added it should be off by default and turned on with `--config-try-environment` or something.

Repro:

```
mb 0 /Users/jetmore/Documents/git/swaks > cat config
--to $TO
--from from@example.com
--helo server.example.com
mb 0 /Users/jetmore/Documents/git/swaks > TO=recip@example.com ./swaks --dump protocol --config ./config
Protocol Info:
protocol = esmtp
helo = server.example.com
from = from@example.com
to = $TO
force getpwuid = FALSE
quit after =
drop after =
drop after send =
server_only = FALSE
timeout = 30
pipeline = FALSE
prdr = FALSE
mb 0 /Users/jetmore/Documents/git/swaks >
```

If implemented, the "to" option in the output should show recip@example.com, not `$TO`

Start by tracing how swaks reads the file passed to `--config` and how the `to` option is parsed, using the provided reproduction with `--dump protocol`. Determine whether environment expansion should be opt-in as proposed. Done means `$TO` in the config resolves to `recip@example.com` only when the new option is enabled, while the default behavior remains unchanged.

Written by the indexing model from the issue text.