Rewrite edns0 (#561)

* 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

test/middleware_dnssec_test.go

@@ -23,7 +23,7 @@ func TestLookupBalanceRewriteCacheDnssec(t *testing.T) {
 
 	corefile := `example.org:0 {
     file ` + name + `
-    rewrite ANY HINFO
+    rewrite type ANY HINFO
     dnssec {
         key file ` + base + `
     }