Please add a "send in plain text" option

[msoulier]

Not all recipients of email, especially those working in open-source, appreciate html email. In fact, many despise it. I like pop but sometimes with automated email, I'm sending snippets of logs and such that should be displayed in a monospace font without losing any of the original whitespace.

Please add this ability to pop with a simple flag and/or environment variable. I had no idea it was going to send me html email until I used it the first time. I was quite surprised.

[jficz]

This PR implements --plaintext flag for SMTP e-mails (but not for Resend API) https://github.com/charmbracelet/pop/pull/125