def self.authorization_url(client_id:, redirect_uri:, scope: [])
opts = {response_type: :code}
opts[:client_id] = client_id
opts[:redirect_uri] = redirect_uri
opts[:scope] = scope.join('+') unless scope.empty?
"#{AUTH_URL}?#{opts.map{|k,v| "#{k}=#{v}"}.join('&')}"
end