API Sprout 1.3.0

• Add --add-server to add a custom server when using --validate-server.
  This allows quickly adding a custom domain or base path that will properly
  validate.
• Add --header (short -H) option to specify a custom header when fetching
  the API document. This allows you to pass custom auth info.
• Add readOnly and writeOnly support to the example generator.
• Revamped support for --validate-server (short -s)
  • Requires the use of server base path(s) on the client.
  • Localhost is now always allowed on all known base paths.
  • Support for proxy headers (e.g. X-Forwarded-Host).
• Better support for resolving relative path references.
• Be more resilient to parser panics when using --watch
• Update Docker build to use Go 1.12 and Go modules.
• Enhance example-from-schema generation code. Support enums, string formats,
  array and object examples, min/max and min items.