Enhancements to Attribute options

[careo]

1. Support array of regular expressions to use for value validation. Either Attribute could accept a list of regular expressions here, or we could provide some way for the type to override the type to override that behavior (so at least String can take an array of regexes).
2. Validate that :values are valid
3. Validate that :default is valid
4. And make the part of check_option below less weird.

Weird part:

unless self.type.valid_type?(definition) || definition.is_a?(::Regexp) || definition.is_a?(::String) || definition.is_a?(::Array)
  raise AttributorException.new("Invalid example type (got: #{definition.class.name}). It must always match the type of the attribute (except if passing Regex that is allowed for some types)")
end