Add support for annotated tags and externalDocs

[mvb-catchsystems]

This adds support for the "tags" element of the root document. This allows tags to have a description and an optional URL for external documentation. This is backwards compatible: Tags which are not explicitly declared via a call to addTag are added to the list of tags (but their description will remain empty).

It also adds support for the "externalDocs" element of the root document.

Example usage

/**
 *  General API docs info
 */
OATPP_CREATE_COMPONENT(std::shared_ptr<oatpp::swagger::DocumentInfo>, swaggerDocumentInfo)([] {

  oatpp::swagger::DocumentInfo::Builder builder;

  builder
  .setTitle("User entity service")
  .setDescription("CRUD API Example project with swagger docs")
  .setVersion("1.0")
  .setContactName("Ivan Ovsyanochka")
  .setContactUrl("https://oatpp.io/")

  .setLicenseName("Apache License, Version 2.0")
  .setLicenseUrl("http://www.apache.org/licenses/LICENSE-2.0")
  .setExternalDocs("https://oatpp.io/docs/modules/oatpp-swagger/", "swagger docs")
  .addServer("http://localhost:8000", "server on localhost")

  .addTag("authentication", "Really important information about the login process")
  .addTag("user", "User-related operations")

  .setExternalDocsForTag("authentication", "https://en.wikipedia.org/wiki/Salt_(cryptography)", "background information")

  return builder.build();

}());

[mvb-catchsystems]

This would fix #85 (well, it fixes the underlying need of that issue)