Allow user to specify licenses

While teaching a training course, it was brought up as a concern that cookbooks all have different licenses. And in some organizations, it may not be acceptable to use a cookbook that falls under a certain license. 

Usage:

```ruby
licenses 'mit', 'apache2' # produces a warning if a cookbook's license is not one of these
```

&nbsp;

```ruby
licenses! 'mit', 'apache2' # raises exception and halts if a cookbook's license is not one of these
```

:+1: if this would be a valuable feature