feat(email): added Metadata to model

[mattevans]

Adds Metadata to the email model to support custom metadata key/value pairs.

https://postmarkapp.com/developer/api/email-api

Usage:

emailReq := &postmark.Email{
  From:       "mail@company.com",
  To:         "jack@sparrow.com",
  TemplateID: 123456,
  TemplateModel: map[string]interface{}{
    "name": "Jack",
    "action_url": "http://click.company.com/welcome",
  },
  Tag:        "onboarding",
  TrackOpens: true,
  Metadata: map[string]string{
    "client-id": "123456",
    "client-ip": "127.0.0.1",
  },
}