Integrating VirusTotal into rspamd

My old setup is using `amavisd-new`, in which I've integrated @ercpe's very useful daemon [amavisvt](https://github.com/ercpe/amavisvt) to check attachments by sending their hashes to [virustotal.com](https://developers.virustotal.com/reference).

It would be nice if `rspamd` could make use of virustotal.com as well.