Wiki Page and Template based default custom displayers

Currently custom displayers are defined in the property of a class, usually calling an include of a wiki page.

This issue is about providing a way to propose a custom displayer in a wiki page or in a template with the same API as current custom displayer but used by default for every field of a specific type in a specific wiki

This would for example allow to bundle in XWiki custom displayers for all date, email or user fields