Implement protection against viruses and phishing

[BjarniRunar]

In addition to spam, viruses and phishing are serious risks that come with receiving e-mail over the Internet. Although Mailpile's built-in spam filter provides a degree of protection against these threats, further steps are necessary, many of which are outlined in our security roadmap.

See also: https://github.com/mailpile/Mailpile/wiki/Security-roadmap