Add hook callback methods.

`BeforeHook()` and `AfterHook()` may be implemented on CLI nodes to
trigger hooks. Use the `Bind()` option to bind potential arguments.

callbacks.go

@@ -0,0 +1,55 @@
+package kong
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type bindings map[reflect.Type]reflect.Value
+
+func (b bindings) add(values ...interface{}) bindings {
+	for _, v := range values {
+		b[reflect.TypeOf(v)] = reflect.ValueOf(v)
+	}
+	return b
+}
+
+// Clone and add values.
+func (b bindings) clone() bindings {
+	out := make(bindings, len(b))
+	for k, v := range b {
+		out[k] = v
+	}
+	return out
+}
+
+func getMethod(value reflect.Value, name string) reflect.Value {
+	method := value.MethodByName(name)
+	if !method.IsValid() {
+		if value.CanAddr() {
+			method = value.Addr().MethodByName(name)
+		}
+	}
+	return method
+}
+
+func callMethod(name string, v, f reflect.Value, bindings bindings) error {
+	in := []reflect.Value{}
+	t := f.Type()
+	if t.NumOut() != 1 || t.Out(0) != callbackReturnSignature {
+		return fmt.Errorf("return value of %T.%s() must be exactly \"error\"", v.Type(), name)
+	}
+	for i := 0; i < t.NumIn(); i++ {
+		pt := t.In(i)
+		if arg, ok := bindings[pt]; ok {
+			in = append(in, arg)
+		} else {
+			return fmt.Errorf("couldn't find binding of type %s for parameter %d of %T.%s(), use kong.Bind(%s)", pt, i, v.Type(), name, pt)
+		}
+	}
+	out := f.Call(in)
+	if out[0].IsNil() {
+		return nil
+	}
+	return out[0].Interface().(error)
+}