Implement RFC 027 File Verification

This implements user-suppliable file content verification per RFC
027. Users can supply a block, string, or symbol to the `verify`
resource attribute.  Blocks will be called, string will be executed as
shell commands (respecting the same options as not_if and only_if), and
symbols can be used to access built-in registered validations.
Can we boil this down a DSL method?

```
register_verification :noop_ruby do |path, opts|
  # yolo
  true
end

register_verification :noop_shell "echo 'yolo'"
```