Mixlib config should have default validation checking

[mmzyk]

It should be possible to state the valid values for a config option and check that either the default value is used or that the option supplied is valid.

This would avoid issues such as the one seen here chef/chef#1509 where cookbook_path was set to nil leading to odd errors.

[KierranM]

Would it also be useful to be able to specify whether a config option is required? I'm picturing a similar system as how attributes are defined in a Chef cookbook LWRP:

require 'mixlib/config'

class ValidatedConfig
  # something like this?
  configurable('string').required.is_a(String)
end