POP3 support

[kiwiz]

Use case

Interfacing with clients that only support POP3 (Gmail, for example)

Your idea for a solution

Adding POP3 support to maddy

• [x] I'm willing to help with the implementation

[foxcpp]

I would simply map POP3 operations to corresponding IMAP ones and reuse same storage interfaces and so on. We need to have a reasonably robust library to decode/encode wire protocol, the rest should be fairly easy.

PRs are welcome, let me know if you need any guidance on where to start.

[foxcpp]

https://github.com/regnull/popgun might be a good starting point.