Add *ready* plugin (#2616)

Add a ready plugin that allows plugin to signal when they are ready.
Once a plugin is ready it is not queried again.

This uses same mechanism as the health plugin: each plugin needs to
implement an interface.

Implement readines for the *erratic* plugin to aid in testing.

Add README.md and tests moduled after the health plugin; which will be
relegated to just providing process health. In similar vein to health
this is a process wide setting.

With this Corefile:
~~~
. {
    erratic
    whoami
    ready
}

bla {
    erratic
    whoami
}
~~~

ready will lead to:

~~~ sh
% curl localhost:8181/ready
% dig @localhost -p 1053 mx example.org
% curl localhost:8181/ready
OK%
~~~

Meanwhile CoreDNS logs:

~~~
.:1053
bla.:1053
2019-02-26T20:59:07.137Z [INFO] CoreDNS-1.3.1
2019-02-26T20:59:07.137Z [INFO] linux/amd64, go1.11.4,
CoreDNS-1.3.1
linux/amd64, go1.11.4,
2019-02-26T20:59:11.415Z [INFO] plugin/ready: Still waiting on: "erratic"
2019-02-26T20:59:13.510Z [INFO] plugin/ready: Still waiting on: "erratic"
~~~

*ready* can be used in multiple server blocks and will do the right
thing; query all those plugins from all server blocks for readiness.
This does a similar thing to the prometheus plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/ready/setup_test.go

@@ -0,0 +1,34 @@
+package ready
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupReady(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+	}{
+		{`ready`, false},
+		{`ready localhost:1234`, false},
+		{`ready localhost:1234 b`, true},
+		{`ready bla`, true},
+		{`ready bla bla`, true},
+	}
+
+	for i, test := range tests {
+		_, err := parse(caddy.NewTestController("dns", test.input))
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found none for input %s", i, 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)
+			}
+		}
+	}
+}