Allow specifying extra parsley attributes in form Meta class

Opening up this issue to start the discussion about this feature proposal.  I may turn this into a pull request soon (once I have some code and tests).

Proposed API:

    @parsleyfy
    class PasswordChangeForm(BasePasswordChangeForm):
        class Meta:
            parsley_extras = {
                'new_password1': {
                    'minlength': "5",
                },
                'new_password2': {
                    'equalto': "new_password1",
                    'error-message': "Your passwords do not match.",
                },
            }