Allow inspecting middlewares

[waiting-for-dev]

In the same way that 8b3e90b did with operations, this commit adds the
option to inspect middlewares once a WebPipe app has been initialized.

Middlewares are returned as
WebPipe::RackSupport::MiddlewareSpecification instances. They are
returned as an array, as they in fact can be a chain of middlewares
built through middleware composition.

require 'web_pipe'
require 'rack/session'

class MyApp
  include WebPipe

  use :session, Rack::Session::Cookie, key: 'my_app.session', secret: 'long'

  plug(:hello) do |conn|
    conn.set_response_body('Hello world!')
  end
end

app = MyApp.new
session_middleware = app.middlewares[:session][0]
session_middleware.middleware # => Rack::Session::Cookie
session_middleware.options # => [{ key: 'my_app.session', secret: 'long' }]