Implement exclusion of tags

Hi there,

Thank you for foodcritic!

We ran into a situation with our twemproxy cookbook where a rule was triggered for wrong reasons:

```ruby
# assuming you perform a Chef search and store it in a variable `nodes`

nodes.each do |node|
      twemproxy_pools ||= {}
      twemproxy_pools[pool] ||= {}
      twemproxy_pools[pool]['twemproxy_port'] = twemproxy_port
      twemproxy_pools[pool]['redis_port'] ||= []
      twemproxy_pools[pool]['redis_port'] << node['pool_info'].grep(/redis.pools.#{env}.#{pool}.\d+/).map { |tag| tag.match(/redis.pools.#{env}.#{pool}.(\d+)/)[1] }[0]
      twemproxy_pools[pool]['nodes'] ||= {}
      twemproxy_pools[pool]['nodes'][node['name']] ||= {}
      twemproxy_pools[pool]['nodes'][node['name']]['ip'] ||= node['ip']
    end
```

In this case, `twemproxy_pools[pool]['nodes'][node['name']]` will trigger `FC039` even though it's "fine" (just shadowing).

Instead of specifying every rule that exists without `FC039`, I wanted a quick way to just exclude one or many, hence this PR.

Let me know if you're open to it and I'll try to write a feature for it.

Thanks for looking into it!