feat(validate): initial implementation

This is a bare-bones implementation of a basic validation package based
around a very simply Validatable interface:

    type Validatable interface {
        Validate() error
    }

The goal is to keep things as simple as possible, while also giving as
much control as possible over how validation logic is performed.

validator_test.go

@@ -0,0 +1,86 @@
+package validate
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+//
+// Test helper types
+//
+
+type testStruct struct {
+	Foo string `json:"foo"`
+
+	f func() error
+}
+
+func (s *testStruct) Validate() error {
+	if s.f == nil {
+		return nil
+	}
+
+	return s.f()
+}
+
+type testNestedStruct struct {
+	OtherField *testStruct `yaml:"other_field"`
+}
+
+type MyStruct struct {
+	Name string
+	Kind string
+}
+
+//
+// Tests
+//
+
+func TestNew(t *testing.T) {
+	got := New()
+
+	assert.IsType(t, &Validator{}, got)
+}
+
+func TestValidator_FieldNameFunc(t *testing.T) {
+	v := New()
+	v.FieldNameFunc(func(sf reflect.StructField) string {
+		return "<" + strings.ToUpper(sf.Name) + ">"
+	})
+	err := v.Validate(&testNestedStruct{
+		OtherField: &testStruct{f: func() error {
+			return &Error{Field: "Foo", Msg: "oops"}
+		}},
+	})
+
+	got := Errors(err)
+
+	assert.ElementsMatch(t, []error{
+		&Error{Field: "<OTHERFIELD>.<FOO>", Msg: "oops"},
+	}, got)
+}
+
+func TestValidator_FieldJoinFunc(t *testing.T) {
+	v := New()
+	v.FieldJoinFunc(func(path []string, field string) string {
+		if field != "" {
+			path = append(path, field)
+		}
+
+		return "[" + strings.Join(path, "][") + "]"
+	})
+	err := v.Validate(&testNestedStruct{
+		OtherField: &testStruct{f: func() error {
+			return &Error{Field: "Foo", Msg: "oops"}
+		}},
+	})
+
+	got := Errors(err)
+
+	assert.ElementsMatch(t, []error{
+		&Error{Field: "[other_field][foo]", Msg: "oops"},
+	}, got)
+}