Error Message Templating

[jimeh]

We want to support some very basic templating features in error messages
returned by Validate() functions.

Specifically, within a Validate() method on a struct type, we want to support
field name normalization via the FieldNameFunc feature, to convert the field
name to it's JSON/YAML/Form tag.

This would allow a error message like:

only allowed ObjectType='book'

The ObjectType field name is the Go struct field name, if this object uses
JSON, the external name of the field in JSON might be object_type or
objectType. So we want to be able to indicate in the error message that
ObjectType is a field on the struct.

I think we could do this a few different ways. Assuming we the following struct:

type Item struct {
	ObjectType string `json:"object_type"`
	BookTitle  string `json:"book_title"`
}

We want the end result to be this:

only allowed object_type='book'

We could use a custom syntax, something like one of these:

only allowed #{ObjectType}='book'

only allowed ${ObjectType}='book'

only allowed {{ObjectType}}='book'

Or we could leverage the text/template package using a custom field
function:

only allowed {{ field "ObjectType" }}='book'

Some of the field validation helpers mentioned in #4 could benefit from this
feature too.