Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

tpl/strings/init_test.go

@@ -1,45 +0,0 @@
-// Copyright 2017 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 strings
-
-import (
-	"testing"
-
-	"github.com/gohugoio/hugo/config"
-
-	"github.com/gohugoio/hugo/htesting/hqt"
-
-	qt "github.com/frankban/quicktest"
-	"github.com/gohugoio/hugo/deps"
-	"github.com/gohugoio/hugo/tpl/internal"
-)
-
-func TestInit(t *testing.T) {
-	c := qt.New(t)
-	var found bool
-	var ns *internal.TemplateFuncsNamespace
-
-	for _, nsf := range internal.TemplateFuncsNamespaceRegistry {
-		ns = nsf(&deps.Deps{Cfg: config.New()})
-		if ns.Name == name {
-			found = true
-			break
-		}
-	}
-
-	c.Assert(found, qt.Equals, true)
-	ctx, err := ns.Context()
-	c.Assert(err, qt.IsNil)
-	c.Assert(ctx, hqt.IsSameType, &Namespace{})
-}