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/health/setup_test.go

@@ -0,0 +1,35 @@
+package health
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupHealth(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+	}{
+		{`health`, false},
+		{`health localhost:1234`, false},
+		{`health bla:a`, false},
+		{`health bla`, true},
+		{`health bla bla`, true},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		_, err := healthParse(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+	}
+}