Remove the word middleware (#1067)

* 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

plugin/errors/setup_test.go

@@ -0,0 +1,45 @@
+package errors
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestErrorsParse(t *testing.T) {
+	tests := []struct {
+		inputErrorsRules     string
+		shouldErr            bool
+		expectedErrorHandler errorHandler
+	}{
+		{`errors`, false, errorHandler{
+			LogFile: "stdout",
+		}},
+		{`errors stdout`, false, errorHandler{
+			LogFile: "stdout",
+		}},
+		{`errors errors.txt`, true, errorHandler{
+			LogFile: "",
+		}},
+		{`errors visible`, true, errorHandler{
+			LogFile: "",
+		}},
+		{`errors { log visible }`, true, errorHandler{
+			LogFile: "stdout",
+		}},
+	}
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.inputErrorsRules)
+		actualErrorsRule, err := errorsParse(c)
+
+		if err == nil && test.shouldErr {
+			t.Errorf("Test %d didn't error, but it should have", i)
+		} else if err != nil && !test.shouldErr {
+			t.Errorf("Test %d errored, but it shouldn't have; got '%v'", i, err)
+		}
+		if actualErrorsRule.LogFile != test.expectedErrorHandler.LogFile {
+			t.Errorf("Test %d expected LogFile to be %s, but got %s",
+				i, test.expectedErrorHandler.LogFile, actualErrorsRule.LogFile)
+		}
+	}
+}