added a responsable class and mail is responsable

[josephmancuso]

[x] Added a responsable class

• Any class that inherits the Responsable class and implements a get_response() method can now be returned in a controller method

[x] Mail classes are now responsable.

You can return a mail template and view the mail in the browser

def show(self, mail: Mail):
    return mail.to('user@email.com').template('some.template', {'key': 'value'})

Closes #891