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

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/aws/aws-sdk-go v1.43.5
 	github.com/bep/debounce v1.2.0
 	github.com/bep/gitmap v1.1.2
+	github.com/bep/goat v0.5.0
 	github.com/bep/godartsass v0.12.0
 	github.com/bep/golibsass v1.0.0
 	github.com/bep/gowebp v0.1.0
@@ -19,7 +20,7 @@ require (
 	github.com/dustin/go-humanize v1.0.0
 	github.com/evanw/esbuild v0.14.22
 	github.com/fortytw2/leaktest v1.3.0
-	github.com/frankban/quicktest v1.14.0
+	github.com/frankban/quicktest v1.14.2
 	github.com/fsnotify/fsnotify v1.5.1
 	github.com/getkin/kin-openapi v0.85.0
 	github.com/ghodss/yaml v1.0.0
@@ -57,7 +58,7 @@ require (
 	github.com/spf13/pflag v1.0.5
 	github.com/tdewolff/minify/v2 v2.9.29
 	github.com/yuin/goldmark v1.4.7
-	github.com/yuin/goldmark-highlighting v0.0.0-20200307114337-60d527fdb691
+	go.uber.org/atomic v1.9.0
 	gocloud.dev v0.20.0
 	golang.org/x/image v0.0.0-20211028202545-6944b10bf410
 	golang.org/x/net v0.0.0-20220127200216-cd36cc0744dd