field constraints support

[vtolstov]

some fields not only must have iso8583 types, but also must conform to some other stuff like len of field, or number of spaces or digits , may be cool to add Validate func(field.Field) error to spec?
So after we fill the fields and Validate func not nil we can call it and get reasonable error if it present?
Also we can create predefined funcs as example for date time fields for example...
i think that this will be cool

[alovak]

@vtolstov it you have a chance to make a draft/proposal for this that would be really helpful.

We can add validation to each field (Numeric, String, Datetime, etc.) or we can add Validation to the Spec. Maybe for the spec we can add something like a format that will be passed to the field validator. What are your thoughts?