Addition of a timer to virustotal checks

[r-tx]

With the free API from VirusTotal, checks are limited to 4 queries a minute and 500 a day, it would useful to have an option to call with --virustotal that would allow the throttling of checks against VT so as not to exhaust the queries.

As an example, Didier Stevens' virustotal-search.py which has a built in 16s delay and can be adjusted with -d/--delay flag