Tag all foodcritic rules with cink

Tagging all the rules with a common string is a vain and trivial change, but increases flexibility. The technique was copied from https://github.com/etsy/foodcritic-rules.

Essentially, it allows the explicit inclusion or exclusion of the rules based on the `cink` tag, as in:

```
$ foodcritic --tags easy,cink,FC023 --include foodcritic/*/rules.rb cookbooks
```

Based on the setup suggested at http://technology.customink.com/blog/2012/08/03/testing-chef-cookbooks/.