Feature: allow users to define their own authentication strategy

[Porcupine96]

It would be nice if a user could define his or hers own authentication strategy.

We could create an abstract class like:

class HTTPBasicAuth
  abstract class AuthenticationStrategy
    abstract def authorize?(username : String, password : String): String?
  end
end

Then a user could write his own implementation:

  class MongoAuthenticationStrategy < AuthenticationStrategy
    def authorize?(username : String, password : String): String?
        # find a user in database and return his ID if it exists
    end
  end

In order to keep backward compatibility we could add another contructor to the HttpBasicAuth

  def initialize(@strategy : AuthenticationStrategy)
    initialize({} of String => String)
  end

and then in the authorize? method perform a check:

    if strategy = @strategy
      strategy.authorize?(username, password)
    else
      @credentials.authorize?(username, password)
    end

What do you think? If you like the idea I can make a pull request and leave it for you to review 😄