add arbitrary validation of attribute values

[cs01]

The attributes.json file builds in validation with key/value pairs, the value being a regular expression. It is desirable to include more complex information as the values of a requirement's attribute, such as yaml, to enable code generation from the requirement itself.

To enforce valid values, the attribute validators would need to allow constraints in addition to regular expressions, such as an arbitrary go callback function that accepts the value as an input (i.e. isValidYaml()).

I am thinking something like "value": {"regexp": ".*"} for regexps, or "value": {"validatorFn": "validYaml"}.

@aleb also suggested that it might make more sense to use make the attributes file a .go file so that function imports can be done natively and caught at compile-time (I like this idea), or consider yaml.