Introduce rate limiting options

[otizonaizit]

Nowadays SMTP servers are more and more strict about how many emails you can send within a session and within a certain time. We should support this in massmail. We need two things:

• limit the number of emails per SMTP session
• limit the send-rate by introducing a time.sleep after sending every single email

Both things should be controllable with a command-line switch, both off by default.