implement DATA command

[milankowww]

This PR adds better support for DATA command. Instead of just replying 250 2.1.0 OK and staying in the command mode, it responds with 354 End data with <CR><LF>.<CR><LF>, receives and logs the e-mail, and only then returns to the command mode again.

Fixes #28

[phin3has]

Thank you @milankowww , I will take a look at this soon. Thank you for your patience.

[phin3has]

@milankowww , apologies, just now getting to this!

I've taken a look at this PR and have a few questions:

1. Why are changes in both mailoney/core.py and src/mailoney/server.py? The main branch only contains mailoney/core.py.
2. Should the honeypot accumulate the full email body? Currently, each TCP segment is logged separately.
3. Have you tested with large emails? Emails that exceed 4096 bytes (the recv() buffer) will be received in multiple chunks.