add support for ULID to swagger:strfmt

[Ilia Pavlov]

Enhancement description

It's great that we have feature like string format for the UUID strings like here:
https://goswagger.io/use/spec/strfmt.html

Recently i've learnt about ULID identity provider:
https://github.com/ulid/spec

My proposal is to add the same helper for the ULID format.

Swagger spec examples

---
definitions:
  user:
    properties:
      id:
        type: string
        format: ulid

---
definitions:
  ULIDType:
    type: string
    format: ulid
  User:
    properties:
      id:
        $ref: '#/definitions/ULIDType'

P.S.

Originally created here: go-swagger#2467