Added: Shared scenario

[niaeashes]

• Add shared annotation for scenario directive: shared scenario is not running when replay scenario.
• Add @use command: it call the another scenario under the nested context.
• Add api.headers config.

example:

shared scenario Signup {
  POST /users {
    username = user-$rand
    password = password
  }
}
scenario Get Profile {
  @use(Signup)
  GET /users/me
}