Add support for (semi-)arbitrary tokens

Imagine a situation where a user wants to send a link to a report every night. The text of the email is static except for the link. Rather than forcing the user to rebuild the full body each night, offer the ability for the user to reference a custom token in the body, and define the custom token on the command line.

```
$ cat body.txt
The result of the night widget report is available and can be viewed at %REPORT_URL%
$ swaks -t report@example.com --body body.txt --token REPORT_URL:http://example.com/report/20210421.html
```

Not convinced about the name of --token or that argument format, but that's the idea

Start by tracing the swaks --body handling and command-line argument parsing described in the issue. Define how a body containing %REPORT_URL% should be expanded from a token argument such as REPORT_URL:http://example.com/report/20210421.html, and consider the unresolved option name and argument format before implementation.

Written by the indexing model from the issue text.