[safety] Support the malwaredomains.com blacklist

malwaredomains.com is a blacklist of known malware domains.
This commit adds support for this blacklist, and a new mode for the module
to preform local lookups only - for private channels where privacy is
important.
[safety] Support simple whitelist configuration

To save API calls to virustotal, users can configure domains which are
"known good". It is recommended to use for frequently posted domain names.