Add support for enumerations as strings.

[RParkerHp]

Currently, if I create an enumeration that's represented by a string in JSON, the generated swagger type is integer.

When I submit that property value as a string in swagger, it silently fails until I change the value to be a number, but then the JSON marshalling fails.

What would be good is to add support for a field tag so the type can be overridden. Something like:

type Model struct {
  type MyType `swagno:"string"`
}

And then:

  fieldType := (field.Tag.Get("swagno"))
  if fieldType == "" {
      fieldType = getType(field.Type.Kind().String())
  }