plugin/cancel: add context cancelation plugin (#2711)

* plugin/cancel: add context cancelation plugin

Per review comments on #2704, move this into a plugin that gets called.
Add the most minimal plugin, tests and documenation.

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

* plugin/cache: add timeout option

review feedback: add option to set custom timeout.

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

* spelling

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

plugin/cancel/setup_test.go

@@ -0,0 +1,29 @@
+package cancel
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	c := caddy.NewTestController("dns", `cancel`)
+	if err := setup(c); err != nil {
+		t.Errorf("Test 1, expected no errors, but got: %q", err)
+	}
+
+	c = caddy.NewTestController("dns", `cancel 5s`)
+	if err := setup(c); err != nil {
+		t.Errorf("Test 2, expected no errors, but got: %q", err)
+	}
+
+	c = caddy.NewTestController("dns", `cancel 5`)
+	if err := setup(c); err == nil {
+		t.Errorf("Test 3, expected errors, but got none")
+	}
+
+	c = caddy.NewTestController("dns", `cancel -1s`)
+	if err := setup(c); err == nil {
+		t.Errorf("Test 4, expected errors, but got none")
+	}
+}