* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
* Add edns0 code rewrite
* check arg count
* change `new`; set EDNS0 if request doesn't have it set
* change set to replace_or_append
* change to append_or_replace
* return error in new
* update documents
* fixt UT
* return error
* go fmt
* Rework for more general EDNS0 use
Also changed how rules are created and validated. Implements
EDNS0 NSID in addition to local.
* go fmt
* README updates, NSID tests and fixes
* gofmt -s -w
* Fix tests for rewrite syntax change
* Add tests, fix error message
* Review nits
* Missed on nit
* More tests, integration test, fix edns0 parse issue
* Fix README, use RewriteIgnored
* go fmt
