tpl/tplimpl: Fix template truth logic

Before this commit, due to a bug in Go's `text/template` package, this would print different output for typed nil interface values:

```
{{ if .AuthenticatedUser }}User is authenticated!{{ else }}{{ end }}
{{ if not .AuthenticatedUser }}{{ else }}}User is authenticated!{{ end }}
```

This commit works around this by wrapping every `if` and `with` with a custom `getif` template func with truth logic that matches `not`, `and` and `or`.

Those 3 template funcs from Go's stdlib are now pulled into Hugo's source tree and adjusted to support custom zero values, e.g. types that implement `IsZero`.

This means that you can now do:

```
{{ with .Date }}{{ . }}{{ end }}
```

And it would work as expected.

Fixes #5738

common/hreflect/helpers_test.go

@@ -0,0 +1,42 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hreflect
+
+import (
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsTruthful(t *testing.T) {
+	assert := require.New(t)
+
+	assert.True(IsTruthful(true))
+	assert.False(IsTruthful(false))
+	assert.True(IsTruthful(time.Now()))
+	assert.False(IsTruthful(time.Time{}))
+}
+
+func BenchmarkIsTruthFul(b *testing.B) {
+	v := reflect.ValueOf("Hugo")
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if !IsTruthfulValue(v) {
+			b.Fatal("not truthful")
+		}
+	}
+}