* Add a template plugin

The template plugin matches the incoming query by class, type and regex
and templates a response with go templates.

* Fix go style errors

* Fix template README example

* Fix corefile example in plugin/template

* Clarify plugin/template/README.md

Add more details and external links where needed.

* Fix code issues in plugin/template

* Add template metrics

* Add section and template to template plugin metrics

* Fix style / remove extra newline on go imports

* Fix typo in plugin/template/README.md

* Update README.md

I've change the format a bit in a PR that I merged yesterday.

* Add authority section to plugin/template

* Fix naming of incoming query name in plugin/template/README.md

* Fix doc syntax in plugin/template/README.md

* Add authority section to plugin/template/README.md config overview

* Add metric labels to plugin/template/README.md metrics section

* Use request.Request to pass state to the template matcher