Add full support for the rules key

[gemfarmer]

Expected behavior:

We would expect that a configuration like so would check to make sure that items within an array were no-dash and lowercase. Rules would be configured in rules.yml and references in a specific schema.

tags:
  type: Array
  one_of: tags.yml
  rules: [no-dash, lowercase]

• check for valid date strings
  • check if date is today's date or later
  • if date is provided, ensure that it matches the filename
• check for valid image path

Actual behavior

It does nothing