feat: Embed() option and Context.Call()

The former allows arbitrary structs to be embedded in the root of the
CLI, with optional tags.

The latter allows an arbitrary function to be called using Kong's
binding functionality.

options_test.go

@@ -58,7 +58,7 @@ func TestInvalidCallback(t *testing.T) {
 	p, err := New(&cli, BindTo(impl("foo"), (*iface)(nil)))
 	assert.NoError(t, err)
 	err = callMethod("method", reflect.ValueOf(impl("??")), reflect.ValueOf(method), p.bindings)
-	assert.EqualError(t, err, `return value of *reflect.rtype.method() must implement "error"`)
+	assert.EqualError(t, err, `kong.impl.method(): return value of func(kong.iface) string must implement "error"`)
 }
 
 type zrror struct{}