Add inspection for unsafe crypto algorithms being imported from stdblib

[dlsniper]

Following the discussion here: https://groups.google.com/forum/?nomobile=true#!topic/golang-dev/jP0vjXdCj2s and here: golang/go#14395 the plugin should provide an inspection for imports of crypto/{des,md5,rc4,sha1} into the current package.
The type should be WEAK_WARNING and it should be enabled by default.