Add clamav package

[navyau09]

This commit adds the clamav to detect viruses, malware & other malicious file threats

[alecthomas]

Very cool! Unfortunately I think this will be a bit more complicated than we would hope, as the referenced tarball is the source. You'll need to do quite a bit of playing around to get Hermit to install the .pkg file in particular. You can check awscli.hcl for an example, or I can take a look at it when I have some spare time.