Ability to force include / not include teiHeader in our response

This is stricto sensu not the most important thing ever. But right now, we do not support any header sending, when I believe people would be interested to do so. Example : list of authorities.

To do so, I'd propose a refsDecl "capitains-adendum" with cRefPattern representing the xpath which shall be transported,

Origin : https://github.com/Capitains/website/issues/13